Make turn diff tracking operation backed (#21180)

## Summary
- replace filesystem-based turn diff tracking with an operation-backed
accumulator
- preserve enough verified apply_patch state to render move-overwrite
cases correctly
- keep the turn/diff/updated contract intact while removing remote-only
turn-diff test skips

This takes the assumption that no 3P services rely on the output format
of `apply_patch`

## Why
For the CCA file system isolation push

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/session/turn.rs

@@ -68,6 +68,7 @@ use codex_analytics::TurnResolvedConfigFact;
 use codex_analytics::build_track_events_context;
 use codex_async_utils::OrCancelExt;
 use codex_features::Feature;
+use codex_git_utils::get_git_repo_root;
 use codex_hooks::HookEvent;
 use codex_hooks::HookEventAfterAgent;
 use codex_hooks::HookPayload;
@@ -365,7 +366,11 @@ pub(crate) async fn run_turn(
     let mut stop_hook_active = false;
     // Although from the perspective of codex.rs, TurnDiffTracker has the lifecycle of a Task which contains
     // many turns, from the perspective of the user, it is a single turn.
-    let turn_diff_tracker = Arc::new(tokio::sync::Mutex::new(TurnDiffTracker::new()));
+    let display_root = get_git_repo_root(turn_context.cwd.as_path())
+        .unwrap_or_else(|| turn_context.cwd.clone().into_path_buf());
+    let turn_diff_tracker = Arc::new(tokio::sync::Mutex::new(TurnDiffTracker::with_display_root(
+        display_root,
+    )));
 
     // `ModelClientSession` is turn-scoped and caches WebSocket + sticky routing state, so we reuse
     // one instance across retries within this turn.
@@ -2247,10 +2252,10 @@ async fn try_run_sampling_request(
 
     if should_emit_turn_diff {
         let unified_diff = {
-            let mut tracker = turn_diff_tracker.lock().await;
+            let tracker = turn_diff_tracker.lock().await;
             tracker.get_unified_diff()
         };
-        if let Ok(Some(unified_diff)) = unified_diff {
+        if let Some(unified_diff) = unified_diff {
             let msg = EventMsg::TurnDiff(TurnDiffEvent { unified_diff });
             sess.clone().send_event(&turn_context, msg).await;
         }