Add agent workflow engine foundation and theme-style page switcher.

Introduce WorkflowEngine with state machine, tool registry, and builtin chat template; migrate stream chat to engine callbacks. Move page mode switching to TopBar actions cluster as a ThemeToggle-style dropdown (聊天/工作室/爽文/房间).

Co-authored-by: Cursor <cursoragent@cursor.com>

data/agent/templates/builtin.chat/template.json

@@ -0,0 +1,16 @@
+{
+  "id": "builtin.chat",
+  "kind": "builtin.chat",
+  "name": "Builtin Chat Reply",
+  "description": "Default single-turn chat workflow migrated from ChatWorkflowService",
+  "version": "1.0.0",
+  "state_machine_path": "state_machine.json",
+  "skills": [
+    {
+      "id": "chat_reply",
+      "name": "Chat Reply",
+      "description": "Main chat reply skill",
+      "path": "skill/chat_reply"
+    }
+  ]
+}