Added support for live updates to skills (#10478)

Add a centralized FileWatcher in codex-core (using notify) that watches
skill roots from the config layer stack (recursive)

Send `SkillsChanged` events when relevant file system changes are
detected

On `SkillsChanged`:
* Invalidate the skills cache immediately in ThreadManager
* Emit EventMsg::SkillsUpdateAvailable to active sessions
~~* Broadcast a new app-server notification:
SkillsListUpdatedNotification~~

This change does not inject new items into the event stream. That means
the agent will not know about new skills, so it won't be able to
implicitly invoke new skills. It also won't know about changes to
existing skills, so if it has already read the contents of a modified
skill, it will not honor the new behavior.

This change also does not detect modifications to AGENTS.md.

I plan to address these limitations in a follow-on PR modeled after
#9985. Injection of new skills and AGENTS was deemed to risky, hence the
need to split the feature into two stages. The changes in this PR were
designed to easily accommodate the second stage once we have some other
foundational changes in place.

Testing: In addition to automated tests, I did manual testing to confirm
that newly-created skills, deleted skills, and renamed skills are
reflected in the TUI skill picker menu. Also confirmed that
modifications to behaviors for explicitly-invoked skills are honored.

---------

Co-authored-by: Xin Lin <xl@openai.com>

codex-rs/core/tests/suite/mod.rs

@@ -81,6 +81,7 @@ mod json_result;
 mod list_dir;
 mod list_models;
 mod live_cli;
+mod live_reload;
 mod model_info_overrides;
 mod model_overrides;
 mod model_tools;