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/lib/types.ts

@@ -44,8 +44,6 @@ export interface UIMessage {
   images?: UIImage[];
   /** Signed or local UI-renderable media attachments. */
   media?: UIMediaAttachment[];
-  /** Optional answer choices for a pending ask_user question. */
-  buttons?: string[][];
 }
 
 export interface ChatSummary {
@@ -141,9 +139,6 @@ export type InboundEvent =
       reply_to?: string;
       media?: string[];
       media_urls?: Array<{ url: string; name?: string }>;
-      buttons?: string[][];
-      /** Original prompt before the websocket text fallback appends buttons. */
-      button_prompt?: string;
       /** Present when the frame is an agent breadcrumb (e.g. tool hint,
        * generic progress line) rather than a conversational reply. */
       kind?: "tool_hint" | "progress";