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/ServerNotification.json

@@ -1485,6 +1485,10 @@
         }
       ]
     },
+    "SkillsChangedNotification": {
+      "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.",
+      "type": "object"
+    },
     "SubAgentSource": {
       "oneOf": [
         {
@@ -3202,6 +3206,26 @@
       "title": "Thread/closedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "skills/changed"
+          ],
+          "title": "Skills/changedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/SkillsChangedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Skills/changedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {