fix: update file search directory when session CWD changes (#9279)

## Summary

Fixes #9041

- Adds update_search_dir() method to FileSearchManager to allow updating
the search directory after initialization
- Calls this method when the session CWD changes: new session, resume,
or fork

## Problem

The FileSearchManager was created once with the initial search_dir and
never updated. When a user:

1. Starts Codex in a non-git directory (e.g., /tmp/random)
2. Resumes or forks a session from a different workspace
3. The @filename lookup still searched the original directory

This caused no matches to be returned even when files existed in the
current workspace.

## Solution

Update FileSearchManager.search_dir whenever the session working
directory changes:
- AppEvent::NewSession: Use current config CWD
- SessionSelection::Resume: Use resumed session CWD
- SessionSelection::Fork: Use forked session CWD

## Test plan

- [ ] Start Codex in /tmp/test-dir (non-git)
- [ ] Resume a session from a project with actual files
- [ ] Verify @filename returns matches from the resumed session
directory

---------

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/tui/src/app.rs

@@ -1370,10 +1370,7 @@ impl App {
                                 self.shutdown_current_thread().await;
                                 self.config = resume_config;
                                 tui.set_notification_method(self.config.tui_notification_method);
-                                self.file_search = FileSearchManager::new(
-                                    self.config.cwd.clone(),
-                                    self.app_event_tx.clone(),
-                                );
+                                self.file_search.update_search_dir(self.config.cwd.clone());
                                 let init = self.chatwidget_init_for_forked_or_resumed_thread(
                                     tui,
                                     self.config.clone(),