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_schemas.py

@@ -447,6 +447,33 @@ FUNCTION_TOOL_SCHEMAS = [
             }
         }
     },
+    {
+        "type": "function",
+        "function": {
+            "name": "ask_user",
+            "description": "Ask the user a multiple-choice question to get a decision or clarification when the task is genuinely ambiguous and the answer changes what you do next (e.g. pick between approaches, confirm an assumption, choose a target). The user sees clickable option buttons; calling this ENDS your turn and their selection arrives as your next message. Prefer sensible defaults over asking — only ask when you truly cannot proceed well without the user's input. Do NOT use it to confirm irreversible/destructive actions that have a dedicated confirmation flow.",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "question": {"type": "string", "description": "The question to ask. Be specific and self-contained."},
+                    "options": {
+                        "type": "array",
+                        "description": "2-6 mutually exclusive choices. Each is an object with a short `label` and an optional `description` explaining the trade-off.",
+                        "items": {
+                            "type": "object",
+                            "properties": {
+                                "label": {"type": "string", "description": "Concise choice text the user clicks (1-5 words)."},
+                                "description": {"type": "string", "description": "Optional one-line explanation of this choice."}
+                            },
+                            "required": ["label"]
+                        }
+                    },
+                    "multi": {"type": "boolean", "description": "Set true to let the user select multiple options instead of one. Default false."}
+                },
+                "required": ["question", "options"]
+            }
+        }
+    },
     {
         "type": "function",
         "function": {