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/components/thread/AskUserPrompt.tsx

@@ -1,108 +0,0 @@
-import { useCallback, useEffect, useRef, useState } from "react";
-import { MessageSquareText } from "lucide-react";
-
-import { Button } from "@/components/ui/button";
-import { cn } from "@/lib/utils";
-
-interface AskUserPromptProps {
-  question: string;
-  buttons: string[][];
-  onAnswer: (answer: string) => void;
-}
-
-export function AskUserPrompt({
-  question,
-  buttons,
-  onAnswer,
-}: AskUserPromptProps) {
-  const [customOpen, setCustomOpen] = useState(false);
-  const [custom, setCustom] = useState("");
-  const inputRef = useRef<HTMLTextAreaElement>(null);
-  const options = buttons.flat().filter(Boolean);
-
-  useEffect(() => {
-    if (customOpen) {
-      inputRef.current?.focus();
-    }
-  }, [customOpen]);
-
-  const submitCustom = useCallback(() => {
-    const answer = custom.trim();
-    if (!answer) return;
-    onAnswer(answer);
-    setCustom("");
-    setCustomOpen(false);
-  }, [custom, onAnswer]);
-
-  if (options.length === 0) return null;
-
-  return (
-    <div
-      className={cn(
-        "mx-auto mb-2 w-full max-w-[49.5rem] rounded-[16px] border border-primary/30",
-        "bg-card/95 p-3 shadow-sm backdrop-blur",
-      )}
-      role="group"
-      aria-label="Question"
-    >
-      <div className="mb-2 flex items-start gap-2">
-        <div className="mt-0.5 rounded-full bg-primary/10 p-1.5 text-primary">
-          <MessageSquareText className="h-3.5 w-3.5" aria-hidden />
-        </div>
-        <p className="min-w-0 flex-1 text-[13.5px] font-medium leading-5 text-foreground">
-          {question}
-        </p>
-      </div>
-
-      <div className="grid gap-1.5 sm:grid-cols-2">
-        {options.map((option) => (
-          <Button
-            key={option}
-            type="button"
-            variant="outline"
-            size="sm"
-            onClick={() => onAnswer(option)}
-            className="justify-start rounded-[10px] px-3 text-left"
-          >
-            <span className="truncate">{option}</span>
-          </Button>
-        ))}
-        <Button
-          type="button"
-          variant="ghost"
-          size="sm"
-          onClick={() => setCustomOpen((open) => !open)}
-          className="justify-start rounded-[10px] px-3 text-muted-foreground"
-        >
-          Other...
-        </Button>
-      </div>
-
-      {customOpen ? (
-        <div className="mt-2 flex gap-2">
-          <textarea
-            ref={inputRef}
-            value={custom}
-            onChange={(event) => setCustom(event.target.value)}
-            onKeyDown={(event) => {
-              if (event.key === "Enter" && !event.shiftKey && !event.nativeEvent.isComposing) {
-                event.preventDefault();
-                submitCustom();
-              }
-            }}
-            rows={1}
-            placeholder="Type your own answer..."
-            className={cn(
-              "min-h-9 flex-1 resize-none rounded-[10px] border border-border/70 bg-background",
-              "px-3 py-2 text-[13.5px] leading-5 outline-none placeholder:text-muted-foreground",
-              "focus-visible:ring-1 focus-visible:ring-primary/40",
-            )}
-          />
-          <Button type="button" size="sm" onClick={submitCustom} disabled={!custom.trim()}>
-            Send
-          </Button>
-        </div>
-      ) : null}
-    </div>
-  );
-}

webui/src/components/thread/ThreadShell.tsx

@@ -13,7 +13,6 @@ import {
 } from "lucide-react";
 import { useTranslation } from "react-i18next";
 
-import { AskUserPrompt } from "@/components/thread/AskUserPrompt";
 import { ThreadComposer } from "@/components/thread/ThreadComposer";
 import { ThreadHeader } from "@/components/thread/ThreadHeader";
 import { StreamErrorNotice } from "@/components/thread/StreamErrorNotice";
@@ -105,21 +104,6 @@ export function ThreadShell({
     dismissStreamError,
   } = useNanobotStream(chatId, initial, hasPendingToolCalls, onTurnEnd);
   const showHeroComposer = messages.length === 0 && !loading;
-  const pendingAsk = useMemo(() => {
-    for (let index = messages.length - 1; index >= 0; index -= 1) {
-      const message = messages[index];
-      if (message.kind === "trace") continue;
-      if (message.role === "user") return null;
-      if (message.role === "assistant" && message.buttons?.some((row) => row.length > 0)) {
-        return {
-          question: message.content,
-          buttons: message.buttons,
-        };
-      }
-      if (message.role === "assistant") return null;
-    }
-    return null;
-  }, [messages]);
 
   useEffect(() => {
     if (!chatId || loading) return;
@@ -247,13 +231,6 @@ export function ThreadShell({
           onDismiss={dismissStreamError}
         />
       ) : null}
-      {pendingAsk ? (
-        <AskUserPrompt
-          question={pendingAsk.question}
-          buttons={pendingAsk.buttons}
-          onAnswer={send}
-        />
-      ) : null}
       {session ? (
         <ThreadComposer
           onSend={send}