Implement :conflicts selector for read_file tool

- Add real git merge conflict extraction matching GJC behavior
- Extract unresolved conflict blocks (<<<<<<, =======, >>>>>>>)
- Support 3-way conflicts with diff3 markers (|||||||)
- Format conflicts as indexed summary with line ranges and labels
- Return clear 'No unresolved git merge conflicts' when clean
- Add comprehensive behavior tests for 2-way, 3-way, and no-conflicts cases
- Update documentation to mark :conflicts as implemented
- Closes #3323

Tests:
- read_with_selector_conflicts_no_conflicts: verifies clean file message
- read_with_selector_conflicts_two_way: verifies multiple conflict detection
- read_with_selector_conflicts_three_way: verifies diff3 marker support

docs/gjc-compat/tool-semantics-phase1.md

@@ -222,7 +222,7 @@ pub fn read_file_in_workspace(
 **Missing**:
 - ✗ Line range selectors (`:L-L+N` syntax)
 - ✗ Raw mode (`:raw`)
-- ✗ Git merge conflict extraction (`:conflicts`)
+- ✓ Git merge conflict extraction (`:conflicts`)
 - ✗ Truncation footers with recovery hints
 - ✗ Structural summarization (collapse braces)
 - ✗ Archive/SQLite support
@@ -272,7 +272,8 @@ pub fn read_file_in_workspace(
 
 ### Batch 1: READ Tool (Priority P0)
 - Add line range selector parsing
-- Implement `:raw` and `:conflicts` modes
+- Implement `:raw` mode
+- ✓ Implement `:conflicts` mode
 - Add truncation footers with recovery hints
 - Implement structural summarization
 - Add tests for all selector combinations

rust/crates/runtime/src/file_ops.rs

@@ -247,6 +247,37 @@ pub fn read_file_with_selector(
     let lines: Vec<&str> = content.lines().collect();
     let total_lines = lines.len();
 
+    // Special handling for conflicts selector
+    if selector == Some("conflicts") {
+        let conflicts = extract_merge_conflicts(&lines);
+        let (selected, num_lines) = if conflicts.is_empty() {
+            (
+                format!("No unresolved git merge conflicts in {}.", path),
+                1,
+            )
+        } else {
+            let display_path = absolute_path
+                .file_name()
+                .and_then(|n| n.to_str())
+                .unwrap_or(path);
+            let summary = format_conflict_summary(&conflicts, display_path);
+            let summary_lines = summary.lines().count();
+            (summary, summary_lines)
+        };
+
+        return Ok(ReadFileOutput {
+            kind: String::from("text"),
+            file: TextFilePayload {
+                file_path: absolute_path.to_string_lossy().into_owned(),
+                content: selected,
+                num_lines,
+                start_line: 1,
+                total_lines,
+            },
+            truncation_footer: None,
+        });
+    }
+
     // Determine which lines to select
     let (start_index, end_index, truncation_footer) = if let Some(sel) = selector {
         // Parse and apply selector
@@ -281,6 +312,168 @@ pub fn read_file_with_selector(
     })
 }
 
+/// Represents a merge conflict block in a file.
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct ConflictBlock {
+    start_line: usize,    // 1-based line number where <<<<<<< starts
+    separator_line: usize, // 1-based line number where ======= is
+    end_line: usize,      // 1-based line number where >>>>>>> is
+    base_line: Option<usize>, // 1-based line number where ||||||| is (for 3-way)
+    ours_label: Option<String>,
+    base_label: Option<String>,
+    theirs_label: Option<String>,
+}
+
+/// Scans lines for unresolved git merge conflict blocks.
+fn extract_merge_conflicts(lines: &[&str]) -> Vec<ConflictBlock> {
+    let mut blocks = Vec::new();
+    let mut i = 0;
+
+    const OURS_PREFIX: &str = "<<<<<<<";
+    const BASE_PREFIX: &str = "|||||||";
+    const SEPARATOR: &str = "=======";
+    const THEIRS_PREFIX: &str = ">>>>>>>";
+
+    while i < lines.len() {
+        let line = lines[i];
+
+        // Check for start of conflict block
+        if let Some(label) = extract_marker_label(line, OURS_PREFIX) {
+            let ours_label = if label.is_empty() { None } else { Some(label.to_string()) };
+            let start_line = i + 1; // Convert to 1-based
+            let mut separator_line = None;
+            let mut base_line = None;
+            let mut base_label = None;
+            let mut end_line = None;
+            let mut theirs_label = None;
+            let mut j = i + 1;
+
+            // Look for separator, optional base marker, and end marker
+            while j < lines.len() {
+                let current = lines[j];
+
+                // Check for base marker (3-way)
+                if let Some(label) = extract_marker_label(current, BASE_PREFIX) {
+                    if base_line.is_none() {
+                        base_line = Some(j + 1);
+                        base_label = if label.is_empty() { None } else { Some(label.to_string()) };
+                        j += 1;
+                        continue;
+                    }
+                }
+
+                // Check for separator
+                if current.trim() == SEPARATOR {
+                    if separator_line.is_none() {
+                        separator_line = Some(j + 1);
+                        j += 1;
+                        continue;
+                    }
+                }
+
+                // Check for end marker
+                if let Some(label) = extract_marker_label(current, THEIRS_PREFIX) {
+                    if separator_line.is_some() {
+                        end_line = Some(j + 1);
+                        theirs_label = if label.is_empty() { None } else { Some(label.to_string()) };
+                        j += 1;
+                        break;
+                    }
+                }
+
+                j += 1;
+            }
+
+            // Only add if we found both separator and end marker
+            if let (Some(sep_line), Some(end)) = (separator_line, end_line) {
+                blocks.push(ConflictBlock {
+                    start_line,
+                    separator_line: sep_line,
+                    end_line: end,
+                    base_line,
+                    ours_label,
+                    base_label,
+                    theirs_label,
+                });
+                i = j;
+                continue;
+            }
+        }
+
+        i += 1;
+    }
+
+    blocks
+}
+
+/// Extract the optional label from a marker line.
+/// Returns the label text (stripped of leading/trailing spaces) if the line matches the prefix,
+/// or None otherwise.
+fn extract_marker_label<'a>(line: &'a str, prefix: &str) -> Option<&'a str> {
+    if line.starts_with(prefix) {
+        let rest = &line[prefix.len()..];
+        // Accept either just the prefix or prefix + single space + label
+        if rest.is_empty() {
+            Some("")
+        } else if rest.starts_with(' ') {
+            Some(rest[1..].trim())
+        } else {
+            None
+        }
+    } else {
+        None
+    }
+}
+
+/// Format conflict blocks as a summary index, like GJC's implementation.
+fn format_conflict_summary(conflicts: &[ConflictBlock], display_path: &str) -> String {
+    let mut output = Vec::new();
+    let total = conflicts.len();
+    let word = if total == 1 { "conflict" } else { "conflicts" };
+
+    output.push(format!("⚠ {} unresolved {} in {}", total, word, display_path));
+
+    // Extract labels for header
+    let ours_label = conflicts.iter().find_map(|c| c.ours_label.as_ref());
+    let theirs_label = conflicts.iter().find_map(|c| c.theirs_label.as_ref());
+    let base_label = conflicts.iter().find_map(|c| c.base_label.as_ref());
+    let any_base = conflicts.iter().any(|c| c.base_line.is_some());
+
+    if let Some(label) = ours_label {
+        output.push(format!("- ours = {}", label));
+    }
+    if let Some(label) = theirs_label {
+        output.push(format!("- theirs = {}", label));
+    }
+    if any_base {
+        let base_name = base_label.map(|l| l.to_string()).unwrap_or_else(|| "(no label)".to_string());
+        output.push(format!("- base = {}", base_name));
+    }
+
+    output.push(
+        "NOTICE: Bulk-resolve with `write({ path: \"conflict://*\", content })`, or address a single block with `write({ path: \"conflict://<N>\", content })`. Inspect a block by reading `conflict://<N>` (add `/ours` / `/theirs` / `/base` for a single side).".to_string()
+    );
+    output.push(
+        "`content` shorthand: `@ours` / `@theirs` / `@base` / `@both` lines expand to the recorded sections; `@both` = ours-then-theirs. Non-token lines pass through verbatim.".to_string()
+    );
+    output.push(String::new());
+
+    // Format each conflict as a one-liner index
+    let id_width = total.to_string().len();
+    for (idx, conflict) in conflicts.iter().enumerate() {
+        let id = idx + 1;
+        let range = if conflict.start_line == conflict.end_line {
+            format!("L{}", conflict.start_line)
+        } else {
+            format!("L{}-{}", conflict.start_line, conflict.end_line)
+        };
+        let kind = if conflict.base_line.is_some() { "  (3-way)" } else { "" };
+        output.push(format!("#{:>width$}  {}{}", id, range, kind, width = id_width));
+    }
+
+    output.join("\n")
+}
+
 /// Apply a parsed line selector to file lines and return the selected range.
 ///
 /// Returns (start_index, end_index, truncation_footer).
@@ -295,7 +488,7 @@ fn apply_line_selector(
             (0, lines.len(), None)
         }
         LineRangeSelector::Conflicts => {
-            // For now, return entire file (git merge conflict extraction would go here)
+            // Conflicts are handled specially in read_file_with_selector, so this shouldn't be reached
             (0, lines.len(), None)
         }
         LineRangeSelector::Lines(ranges) => {
@@ -1400,4 +1593,73 @@ mod tests {
         );
         assert!(result.is_err());
     }
+
+    #[test]
+    fn read_with_selector_conflicts_no_conflicts() {
+        let path = temp_path("conflicts-none.txt");
+        write_file(
+            path.to_string_lossy().as_ref(),
+            "const a = 1;\nconst b = 2;\n",
+        )
+        .expect("write should succeed");
+
+        let output = super::read_file_with_selector(
+            path.to_string_lossy().as_ref(),
+            None,
+            None,
+            Some("conflicts"),
+        )
+        .expect("read with conflicts selector should succeed");
+        assert!(output.file.content.contains("No unresolved git merge conflicts"));
+    }
+
+    #[test]
+    fn read_with_selector_conflicts_two_way() {
+        let path = temp_path("conflicts-two-way.txt");
+        let content = "<<<<<<< A\na-ours\n=======\na-theirs\n>>>>>>> A\nmiddle\n<<<<<<< B\nb-ours\n=======\nb-theirs\n>>>>>>> B\ntail";
+        write_file(path.to_string_lossy().as_ref(), content)
+            .expect("write should succeed");
+
+        let output = super::read_file_with_selector(
+            path.to_string_lossy().as_ref(),
+            None,
+            None,
+            Some("conflicts"),
+        )
+        .expect("read with conflicts selector should succeed");
+
+        let text = &output.file.content;
+        // Should report 2 conflicts
+        assert!(text.contains("2 unresolved conflicts"), "text: {}", text);
+        // Should have indices for both blocks
+        assert!(text.contains("#1"), "missing #1: {}", text);
+        assert!(text.contains("#2"), "missing #2: {}", text);
+        // Should have line ranges
+        assert!(text.contains("L1-5"), "missing L1-5: {}", text);
+        assert!(text.contains("L7-11"), "missing L7-11: {}", text);
+    }
+
+    #[test]
+    fn read_with_selector_conflicts_three_way() {
+        let path = temp_path("conflicts-three-way.txt");
+        let content = "head\n<<<<<<< HEAD\nours body\n||||||| common ancestor\nbase body\n=======\ntheirs body\n>>>>>>> feat\ntail";
+        write_file(path.to_string_lossy().as_ref(), content)
+            .expect("write should succeed");
+
+        let output = super::read_file_with_selector(
+            path.to_string_lossy().as_ref(),
+            None,
+            None,
+            Some("conflicts"),
+        )
+        .expect("read with conflicts selector should succeed");
+
+        let text = &output.file.content;
+        // Should report 1 conflict
+        assert!(text.contains("1 unresolved conflict"), "text: {}", text);
+        // Should mark as 3-way
+        assert!(text.contains("3-way"), "missing 3-way marker: {}", text);
+        // Should show line range
+        assert!(text.contains("L2-8"), "missing L2-8: {}", text);
+    }
 }