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)

webui/src/hooks/useNanobotStream.ts

@@ -230,7 +230,7 @@ export function useNanobotStream(
         // the full turn (all tool calls + final text) is complete.
         setMessages((prev) => {
           const filtered = activeId ? prev.filter((m) => m.id !== activeId) : prev;
-          const content = ev.buttons?.length ? (ev.button_prompt ?? ev.text) : ev.text;
+          const content = ev.text;
           return [
             ...filtered,
             {
@@ -238,7 +238,6 @@ export function useNanobotStream(
               role: "assistant",
               content,
               createdAt: Date.now(),
-              ...(ev.buttons && ev.buttons.length > 0 ? { buttons: ev.buttons } : {}),
               ...(hasMedia ? { media } : {}),
             },
           ];