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>

backend/services/tool_registry.py

@@ -0,0 +1,49 @@
+"""
+Tool registry for workflow engine steps.
+"""
+from __future__ import annotations
+
+from typing import Any, Awaitable, Callable, Dict, Optional
+
+try:
+    from backend.models.agent import TurnContext, ToolSpec
+except ImportError:
+    from models.agent import TurnContext, ToolSpec
+
+ToolHandler = Callable[[TurnContext], Awaitable[None]]
+
+
+class ToolRegistry:
+    def __init__(self) -> None:
+        self._tools: Dict[str, ToolHandler] = {}
+        self._specs: Dict[str, ToolSpec] = {}
+
+    def register(
+        self,
+        name: str,
+        handler: ToolHandler,
+        *,
+        description: str = "",
+        parameters: Optional[Dict[str, Any]] = None,
+    ) -> None:
+        self._tools[name] = handler
+        self._specs[name] = ToolSpec(
+            name=name,
+            description=description,
+            parameters=parameters or {},
+        )
+
+    def get(self, name: str) -> ToolHandler:
+        if name not in self._tools:
+            raise KeyError(f"Unknown tool: {name}")
+        return self._tools[name]
+
+    def list_specs(self) -> list[ToolSpec]:
+        return list(self._specs.values())
+
+    async def execute(self, name: str, ctx: TurnContext) -> None:
+        handler = self.get(name)
+        await handler(ctx)
+
+
+default_tool_registry = ToolRegistry()