Move file watcher out of core (#21290)

## Why

The app-server watcher relocation leaves the generic filesystem watcher
as the last watcher-specific implementation still living inside
`codex-core`. Moving that code to a small crate keeps `codex-core`
focused on thread execution and lets app-server depend on the watcher
without reaching back into core for filesystem watching primitives.

This PR is stacked on #21287.

## What changed

- Added a new `codex-file-watcher` crate containing the existing watcher
implementation and its unit tests.
- Updated app-server `fs_watch`, `skills_watcher`, and listener state to
import watcher types from `codex-file-watcher`.
- Removed the `file_watcher` module and `notify` dependency from
`codex-core`.
- Updated Cargo workspace metadata and `Cargo.lock` for the new internal
crate.

## Validation

- `cargo check -p codex-file-watcher -p codex-core -p codex-app-server`
- `cargo test -p codex-file-watcher`
- `cargo test -p codex-app-server
skills_changed_notification_is_emitted_after_skill_change`
- `just bazel-lock-update`
- `just bazel-lock-check`
- `just fix -p codex-file-watcher`
- `just fix -p codex-core`
- `just fix -p codex-app-server`

codex-rs/app-server/src/thread_state.rs

@@ -7,7 +7,7 @@ use codex_app_server_protocol::Turn;
 use codex_app_server_protocol::TurnError;
 use codex_core::CodexThread;
 use codex_core::ThreadConfigSnapshot;
-use codex_core::file_watcher::WatchRegistration;
+use codex_file_watcher::WatchRegistration;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::RolloutItem;