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/tests/thread-shell.test.tsx

@@ -809,46 +809,4 @@ describe("ThreadShell", () => {
     await waitFor(() => expect(screen.getByText("from chat b")).toBeInTheDocument());
     expect(screen.queryByText("from chat a")).not.toBeInTheDocument();
   });
-
-  it("renders ask_user options above the composer and sends selected answers", async () => {
-    const client = makeClient();
-    const onNewChat = vi.fn().mockResolvedValue("chat-a");
-
-    render(
-      wrap(
-        client,
-        <ThreadShell
-          session={session("chat-a")}
-          title="Chat chat-a"
-          onToggleSidebar={() => {}}
-          onGoHome={() => {}}
-          onNewChat={onNewChat}
-        />,
-      ),
-    );
-
-    await act(async () => {
-      client._emitChat("chat-a", {
-        event: "message",
-        chat_id: "chat-a",
-        text: "How should I continue?",
-        buttons: [["Short answer", "Detailed answer"]],
-      });
-    });
-
-    expect(screen.getByRole("group", { name: "Question" })).toHaveTextContent(
-      "How should I continue?",
-    );
-
-    fireEvent.click(screen.getByRole("button", { name: "Short answer" }));
-
-    expect(client.sendMessage).toHaveBeenCalledWith(
-      "chat-a",
-      "Short answer",
-      undefined,
-    );
-    await waitFor(() => {
-      expect(screen.queryByRole("group", { name: "Question" })).not.toBeInTheDocument();
-    });
-  });
 });