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/codex_app_server_protocol.schemas.json

@@ -6039,6 +6039,26 @@
           "title": "Thread/closedNotification",
           "type": "object"
         },
+        {
+          "properties": {
+            "method": {
+              "enum": [
+                "skills/changed"
+              ],
+              "title": "Skills/changedNotificationMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/SkillsChangedNotification"
+            }
+          },
+          "required": [
+            "method",
+            "params"
+          ],
+          "title": "Skills/changedNotification",
+          "type": "object"
+        },
         {
           "properties": {
             "method": {
@@ -12302,6 +12322,12 @@
         ],
         "type": "object"
       },
+      "SkillsChangedNotification": {
+        "$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"
+      },
       "SkillsConfigWriteParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "properties": {