This adds a first-class app-server v2 `skills/changed` notification for
the existing skills live-reload signal.
Before this change, clients only had the legacy raw
`codex/event/skills_update_available` event. With this PR, v2 clients
can listen for a typed JSON-RPC notification instead of depending on the
legacy `codex/event/*` stream, which we want to remove soon.
Owen Lin