feat(ai): name agents after their model, not hardcoded "oracle"

Agents now join under their model tag (e.g. "qwen2.5:3b" — model name +
parameter size) or profile label, derived at /ai start and tracked on
App.agent_name, so the roster shows what's actually answering. Updates the
broker re-grant + /ai stop revoke paths, the Python --name default
(falls back to provider.model), and the demo/bootstrap/README/docs refs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.MD

@@ -275,7 +275,8 @@ directly:
 
 ```bash
 .venv/bin/python -m cmd_chat.agent <server_ip> <port> \
-    --name oracle --password <room-pw> --provider ollama --model qwen2.5:3b
+    --password <room-pw> --provider ollama --model qwen2.5:3b
+# joins as its model tag ("qwen2.5:3b") unless you override with --name
 # cloud (opt-in): --provider anthropic --model claude-opus-4-6   (needs ANTHROPIC_API_KEY)
 ```