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/tools/test_tool_loader.py

@@ -405,7 +405,7 @@ def test_loader_registers_same_tools_as_old_hardcoded():
     registered = loader.load(ctx, registry)
 
     expected = {
-        "ask_user", "read_file", "write_file", "edit_file", "list_dir",
+        "read_file", "write_file", "edit_file", "list_dir",
         "glob", "grep", "notebook_edit", "exec", "web_search", "web_fetch",
         "message", "spawn", "cron",
     }