feat: show mock agent hint bubble in empty state

docs/building-chat-ui.mdx

@@ -22,6 +22,7 @@ Capabilities tell you which features are supported for the selected agent:
 - `questions` and `permissions` indicate HITL flows.
 - `plan_mode` indicates that the agent supports plan-only execution.
 - `reasoning` and `status` indicate that the agent can emit reasoning/status content parts.
+- `item_started` indicates that the agent emits `item.started` on its own; when false the daemon will emit a synthetic `item.started` immediately after sending a user message.
 
 Use these to enable or disable UI affordances (tool panels, approval buttons, etc.).
 

docs/openapi.json

@@ -668,6 +668,7 @@
           "fileChanges",
           "mcpTools",
           "streamingDeltas",
+          "itemStarted",
           "sharedProcess"
         ],
         "properties": {
@@ -686,6 +687,9 @@
           "images": {
             "type": "boolean"
           },
+          "itemStarted": {
+            "type": "boolean"
+          },
           "mcpTools": {
             "type": "boolean"
           },