add @plugin mentions (#13510)

## Note-- added plugin mentions via @, but that conflicts with file
mentions

depends and builds upon #13433.

- introduces explicit `@plugin` mentions. this injects the plugin's mcp
servers, app names, and skill name format into turn context as a dev
message.
- we do not yet have UI for these mentions, so we currently parse raw
text (as opposed to skills and apps which have UI chips, autocomplete,
etc.) this depends on a `plugins/list` app-server endpoint we can feed
the UI with, which is upcoming
- also annotate mcp and app tool descriptions with the plugin(s) they
come from. this gives the model a first class way of understanding what
tools come from which plugins, which will help implicit invocation.

### Tests
Added and updated tests, unit and integration. Also confirmed locally a
raw `@plugin` injects the dev message, and the model knows about its
apps, mcps, and skills.

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

@@ -119,6 +119,13 @@
         },
         "name": {
           "type": "string"
+        },
+        "pluginDisplayNames": {
+          "default": [],
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
         }
       },
       "required": [