## Summary
`fs/watch` was using a local debounce wrapper whose deadline was
initialized once and then reused after the first batch. Once that stale
deadline was in the past, later file changes could bypass the intended
200ms debounce and send noisier `fs/changed` notifications.
This moves the debounce wrapper into `codex-file-watcher` as
`DebouncedWatchReceiver`, resets the debounce deadline for each event
batch, preserves pending paths across cancelled receives, and updates
app-server `fs/watch` to use the shared wrapper.
Fixes#24692.
## 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`
