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)

nanobot/channels/websocket.py

@@ -55,14 +55,6 @@ def _normalize_config_path(path: str) -> str:
     return _strip_trailing_slash(path)
 
 
-def _append_buttons_as_text(text: str, buttons: list[list[str]]) -> str:
-    labels = [label for row in buttons for label in row if label]
-    if not labels:
-        return text
-    fallback = "\n".join(f"{index}. {label}" for index, label in enumerate(labels, 1))
-    return f"{text}\n\n{fallback}" if text else fallback
-
-
 class WebSocketConfig(Base):
     """WebSocket server channel configuration.
 
@@ -1468,16 +1460,11 @@ class WebSocketChannel(BaseChannel):
             await self.send_session_updated(msg.chat_id)
             return
         text = msg.content
-        if msg.buttons:
-            text = _append_buttons_as_text(text, msg.buttons)
         payload: dict[str, Any] = {
             "event": "message",
             "chat_id": msg.chat_id,
             "text": text,
         }
-        if msg.buttons:
-            payload["buttons"] = msg.buttons
-            payload["button_prompt"] = msg.content
         if msg.media:
             payload["media"] = msg.media
             urls: list[dict[str, str]] = []