Preserve resumable history when clearing CLI sessions

PARITY.md and the current Rust CLI UX both pointed at session-management polish as a worthwhile parity lane. The existing /clear flow reset the live REPL without telling the user how to get back, and the resumed /clear path overwrote the saved session file in place with no recovery handle.

This change keeps the existing clear semantics but makes them safer and more legible. Live clears now print the previous session id and a resume hint, while resumed clears write a sibling backup before resetting the requested session file and report both the backup path and the new session id.

Constraint: Keep /clear compatible with follow-on commands in the same --resume invocation
Rejected: Switch resumed /clear to a brand-new primary session path | would break the expected in-place reset semantics for chained resume commands
Confidence: high
Scope-risk: narrow
Directive: Preserve explicit recovery hints in /clear output if session lifecycle behavior changes again
Tested: cargo test --manifest-path rust/Cargo.toml -p rusty-claude-cli --test resume_slash_commands
Tested: cargo test --manifest-path rust/Cargo.toml -p rusty-claude-cli --bin claw clear_command_requires_explicit_confirmation_flag
Not-tested: Manual interactive REPL /clear run

rust/crates/rusty-claude-cli/tests/resume_slash_commands.rs

@@ -5,6 +5,7 @@ use std::process::{Command, Output};
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::time::{SystemTime, UNIX_EPOCH};
 
+use runtime::ContentBlock;
 use runtime::Session;
 
 static TEMP_COUNTER: AtomicU64 = AtomicU64::new(0);
@@ -51,7 +52,12 @@ fn resumed_binary_accepts_slash_commands_with_arguments() {
     assert!(stdout.contains("Export"));
     assert!(stdout.contains("wrote transcript"));
     assert!(stdout.contains(export_path.to_str().expect("utf8 path")));
-    assert!(stdout.contains("Cleared resumed session file"));
+    assert!(stdout.contains("Session cleared"));
+    assert!(stdout.contains("Mode             resumed session reset"));
+    assert!(stdout.contains("Previous session"));
+    assert!(stdout.contains("Resume previous  claw --resume"));
+    assert!(stdout.contains("Backup           "));
+    assert!(stdout.contains("Session file     "));
 
     let export = fs::read_to_string(&export_path).expect("export file should exist");
     assert!(export.contains("# Conversation Export"));
@@ -59,6 +65,18 @@ fn resumed_binary_accepts_slash_commands_with_arguments() {
 
     let restored = Session::load_from_path(&session_path).expect("cleared session should load");
     assert!(restored.messages.is_empty());
+
+    let backup_path = stdout
+        .lines()
+        .find_map(|line| line.strip_prefix("  Backup           "))
+        .map(PathBuf::from)
+        .expect("clear output should include backup path");
+    let backup = Session::load_from_path(&backup_path).expect("backup session should load");
+    assert_eq!(backup.messages.len(), 1);
+    assert!(matches!(
+        backup.messages[0].blocks.first(),
+        Some(ContentBlock::Text { text }) if text == "ship the slash command harness"
+    ));
 }
 
 #[test]