feat(app-server): add a skills/changed v2 notification (#13414)

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.

codex-rs/app-server-protocol/schema/json/v2/SkillsChangedNotification.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Notification emitted when watched local skill files change.\n\nTreat this as an invalidation signal and re-run `skills/list` with the client's current parameters when refreshed skill metadata is needed.",
+  "title": "SkillsChangedNotification",
+  "type": "object"
+}