Add ask_user tool: agent-posed multiple-choice questions (#2111)

Let the agent pause and ask the user a multiple-choice question when a
task is genuinely ambiguous and the answer changes what it does next —
choosing between approaches, confirming an assumption, picking a target —
instead of guessing.

Modeled on the existing `ui_control` marker pattern: the `ask_user` tool
returns an `ask_user` payload that the agent loop emits as an SSE event
and then ends the turn. The frontend renders the question with clickable
option buttons, a free-text "Other" input, and an x to dismiss; the user's
choice is sent as the next message and the agent resumes with it in
context.

- src/tool_execution.py: `ask_user` handler — pure UI marker, no I/O.
  Validates a non-empty question + 2..6 options, normalizes string/object
  options, returns the payload.
- src/agent_loop.py: emit the `ask_user` event and break the round loop so
  the turn ends and waits for the user's selection. Stream the question as
  assistant text so it persists/replays (prevents a re-ask loop).
- Registration: TOOL_TAGS, ALWAYS_AVAILABLE, BUILTIN_TOOL_DESCRIPTIONS,
  FUNCTION_TOOL_SCHEMAS, the system-prompt blurb. Not admin-gated (any
  user can be asked); the structured args serialize via the default
  json.dumps path.
- routes/chat_routes.py: relay the `ask_user` event to the client.
- static/js/chat.js + static/style.css: render the question card (options +
  free-text Other + dismiss x; removed once answered). Reuses CSS vars and
  the .modal-close button; emoji go through the monochrome-SVG pipeline.
  Bump chat.js cache pin.
- tests/test_ask_user_tool.py: payload, multi flag, string options, option
  cap, validation errors, serializer round-trip, registration.

src/tool_index.py

@@ -52,6 +52,9 @@ ALWAYS_AVAILABLE = frozenset({
     # of topic. Without this, RAG drops it and the agent falls back to
     # app_api /api/memory/add which fails with 422 on first attempt.
     "manage_memory",
+    # Ask the user a multiple-choice question for a decision/clarification.
+    # Always reachable so the agent can pause and ask at any point.
+    "ask_user",
 })
 
 # Tools that the Personal Assistant always has access to during scheduled
@@ -111,6 +114,7 @@ BUILTIN_TOOL_DESCRIPTIONS: Dict[str, str] = {
     "list_sessions": "List all chats with their metadata (the UI calls these 'chats'). Use for 'list my chats', 'rename all my chats' (list first, then manage_session to rename each).",
     "send_to_session": "Send a message to another chat. Cross-chat communication.",
     "search_chats": "Search through chat history across all sessions.",
+    "ask_user": "Ask the user a multiple-choice question to get a decision or clarification. Use this when the task is genuinely ambiguous and the answer changes what you do next — pick between approaches, confirm an assumption, choose among options — instead of guessing. Provide a clear `question` and 2-6 `options` (each with a short `label`, optional `description`). Calling this ENDS your turn: the user sees clickable buttons and their choice arrives as your next message. Don't use it for things you can decide from context or sensible defaults, or for irreversible-action confirmation if a dedicated flow exists.",
     "ui_control": "Control the UI and toggle tools on/off. Use this to turn off / turn on / disable / enable individual tools and features: shell (bash), search (web), research, browser, documents, incognito. Open panels (documents library, gallery, email inbox, sessions, notes, memories/brain, skills, settings, cookbook) via `open_panel <name>`. Use `open_email_reply <uid> <folder> reply` to open an email reply draft document without sending. Also switches between chat/agent modes, changes the current model, and applies/creates themes.",
     "list_email_accounts": "List configured email accounts and default status. Use before reading or sending mail when the user mentions Gmail, work mail, custom domain mail, another mailbox, or asks to compare/check multiple inboxes.",
     "list_emails": "List emails for a folder/account, newest first, including read messages by default. Shows subject, sender, date, UID, account, and AI summary. Check inbox, find emails needing replies. Supports account from list_email_accounts for Gmail/work/custom mailboxes. For last/latest/newest email, use max_results=1 and unread_only=false.",