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/state_machine.json

@@ -0,0 +1,41 @@
+{
+  "initial": "regex_apply_user_input",
+  "states": {
+    "regex_apply_user_input": {
+      "tool": "regex_apply_user_input",
+      "next": "load_character"
+    },
+    "load_character": {
+      "tool": "load_character",
+      "next": "activate_worldbook"
+    },
+    "activate_worldbook": {
+      "tool": "activate_worldbook",
+      "next": "load_chat_history"
+    },
+    "load_chat_history": {
+      "tool": "load_chat_history",
+      "next": "build_prompt_messages"
+    },
+    "build_prompt_messages": {
+      "tool": "build_prompt_messages",
+      "next": "llm_main_reply"
+    },
+    "llm_main_reply": {
+      "tool": "llm_main_reply",
+      "next": "regex_apply_ai_output"
+    },
+    "regex_apply_ai_output": {
+      "tool": "regex_apply_ai_output",
+      "next": "record_token_usage"
+    },
+    "record_token_usage": {
+      "tool": "record_token_usage",
+      "next": "enqueue_parallel_tasks"
+    },
+    "enqueue_parallel_tasks": {
+      "tool": "enqueue_parallel_tasks",
+      "next": "end"
+    }
+  }
+}