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/Cargo.lock

@@ -1900,6 +1900,7 @@ dependencies = [
  "codex-features",
  "codex-feedback",
  "codex-file-search",
+ "codex-file-watcher",
  "codex-git-utils",
  "codex-hooks",
  "codex-login",
@@ -2551,7 +2552,6 @@ dependencies = [
  "insta",
  "libc",
  "maplit",
- "notify",
  "once_cell",
  "openssl-sys",
  "opentelemetry",
@@ -2894,6 +2894,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "codex-file-watcher"
+version = "0.0.0"
+dependencies = [
+ "notify",
+ "pretty_assertions",
+ "tempfile",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "codex-git-utils"
 version = "0.0.0"