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/slack.py

@@ -471,7 +471,7 @@ class SlackChannel(BaseChannel):
         return preview.startswith(_HTML_DOWNLOAD_PREFIXES)
 
     async def _on_block_action(self, client: SocketModeClient, req: SocketModeRequest) -> None:
-        """Handle button clicks from ask_user blocks."""
+        """Handle button clicks from inline action buttons."""
         await client.send_socket_mode_response(SocketModeResponse(envelope_id=req.envelope_id))
         payload = req.payload or {}
         actions = payload.get("actions") or []
@@ -568,7 +568,7 @@ class SlackChannel(BaseChannel):
 
     @staticmethod
     def _build_button_blocks(text: str, buttons: list[list[str]]) -> list[dict[str, Any]]:
-        """Build Slack Block Kit blocks with action buttons for ask_user choices."""
+        """Build Slack Block Kit blocks with action buttons."""
         blocks: list[dict[str, Any]] = [
             {"type": "section", "text": {"type": "mrkdwn", "text": text[:3000]}},
         ]
@@ -579,7 +579,7 @@ class SlackChannel(BaseChannel):
                     "type": "button",
                     "text": {"type": "plain_text", "text": label[:75]},
                     "value": label[:75],
-                    "action_id": f"ask_user_{label[:50]}",
+                    "action_id": f"btn_{label[:50]}",
                 })
         if elements:
             blocks.append({"type": "actions", "elements": elements[:25]})