docs(ag-ui): regenerate api-docs for AgUiAgent.subagents field (#670)

The F5 PR added a documented subagents signal to AgUiAgent but didn't
regenerate the website api-docs; the Website build drift-guard failed on main.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: blove

apps/website/content/docs/ag-ui/api/api-docs.json

@@ -476,8 +476,8 @@
       {
         "name": "subagents",
         "type": "Signal<Map<string, Subagent>>",
-        "description": "",
-        "optional": true
+        "description": "Subagent activities (activityType==='subagent') projected to the neutral\n Subagent contract, keyed by messageId. Narrows the base Agent's optional\n `subagents?` to a concrete signal for AG-UI consumers.",
+        "optional": false
       },
       {
         "name": "submit",