refactor(agent): remove ask_user tool

The ask_user tool used AskUserInterrupt(BaseException) for mid-turn
blocking, creating heavy coupling across runner, loop, and session
management. The model now asks questions naturally in response text,
the turn ends normally, and the user's next message starts a new turn
with session history providing continuity.

Removed:
- nanobot/agent/tools/ask.py (tool, interrupt, helpers)
- tests/agent/test_ask_user.py
- webui/src/components/thread/AskUserPrompt.tsx
- AskUserInterrupt handling in runner.py
- Dual-path message building in loop.py
- Pending ask detection via history scanning
- button_prompt/buttons emission in WebSocket channel
- ask_user references in Slack channel docstrings

Preserved (MessageTool uses these independently):
- OutboundMessage.buttons field
- Channel button rendering (Telegram, Slack, WebSocket)

tests/channels/test_websocket_channel.py

@@ -224,11 +224,9 @@ async def test_send_delivers_json_message_with_media_and_reply() -> None:
     payload = json.loads(mock_ws.send.call_args[0][0])
     assert payload["event"] == "message"
     assert payload["chat_id"] == "chat-1"
-    assert payload["text"] == "hello\n\n1. Yes\n2. No"
-    assert payload["button_prompt"] == "hello"
+    assert payload["text"] == "hello"
     assert payload["reply_to"] == "m1"
     assert payload["media"] == ["/tmp/a.png"]
-    assert payload["buttons"] == [["Yes", "No"]]
 
 
 @pytest.mark.asyncio