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/agent_tools.py

@@ -34,7 +34,7 @@ TOOL_TAGS = {"bash", "python", "web_search", "web_fetch", "read_file", "write_fi
              "send_to_session",
              "pipeline",
              "manage_session", "manage_memory", "list_models",
-             "ui_control", "generate_image",
+             "ui_control", "generate_image", "ask_user",
              "manage_tasks", "api_call", "ask_teacher", "manage_skills",
              "suggest_document",
              "manage_endpoints", "manage_mcp", "manage_webhooks",