fix(cli): /diff shows clear error when not in a git repo

Previously claw --resume <session> /diff would produce:
  'git diff --cached failed: error: unknown option `cached\''
when the CWD was not inside a git project, because git falls back to
--no-index mode which does not support --cached.

Two fixes:
1. render_diff_report_for() checks 'git rev-parse --is-inside-work-tree'
   before running git diff, and returns a human-readable message if not
   in a git repo:
     'Diff\n  Result  no git repository\n  Detail  <cwd> is not inside a git project'
2. resume /diff now uses std::env::current_dir() instead of the session
   file's parent directory as the CWD for the diff (session parent dir
   is the .claw/sessions/<id>/ directory, never a git repo).

159 CLI tests pass, fmt clean.

rust/crates/rusty-claude-cli/src/main.rs

@@ -2708,7 +2708,7 @@ fn run_resume_command(
         SlashCommand::Diff => Ok(ResumeCommandOutcome {
             session: session.clone(),
             message: Some(render_diff_report_for(
-                session_path.parent().unwrap_or_else(|| Path::new(".")),
+                &std::env::current_dir().unwrap_or_else(|_| PathBuf::from(".")),
             )?),
             json: None,
         }),
@@ -5242,6 +5242,21 @@ fn render_diff_report() -> Result<String, Box<dyn std::error::Error>> {
 }
 
 fn render_diff_report_for(cwd: &Path) -> Result<String, Box<dyn std::error::Error>> {
+    // Verify we are inside a git repository before calling `git diff`.
+    // Running `git diff --cached` outside a git tree produces a misleading
+    // "unknown option `cached`" error because git falls back to --no-index mode.
+    let in_git_repo = std::process::Command::new("git")
+        .args(["rev-parse", "--is-inside-work-tree"])
+        .current_dir(cwd)
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false);
+    if !in_git_repo {
+        return Ok(format!(
+            "Diff\n  Result           no git repository\n  Detail           {} is not inside a git project",
+            cwd.display()
+        ));
+    }
     let staged = run_git_diff_command_in(cwd, &["diff", "--cached"])?;
     let unstaged = run_git_diff_command_in(cwd, &["diff"])?;
     if staged.trim().is_empty() && unstaged.trim().is_empty() {