feat: implement lazy sandbox and thread data initialization (#11)

Defer sandbox acquisition and thread directory creation until first use to improve performance and reduce resource usage.

Changes:
- Add lazy_init parameter to SandboxMiddleware (default: true)
- Add ensure_sandbox_initialized() helper for lazy sandbox acquisition
- Update all sandbox tools to use lazy initialization
- Add lazy_init parameter to ThreadDataMiddleware (default: true)
- Create thread directories on-demand in AioSandboxProvider
- LocalSandbox already creates directories on write (no changes needed)

Benefits:
- Saves 1-2s Docker container startup for conversations without tools
- Reduces unnecessary directory creation and file system operations
- Backward compatible with lazy_init=false option

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/sandbox/middleware.py

@@ -19,7 +19,8 @@ class SandboxMiddleware(AgentMiddleware[SandboxMiddlewareState]):
     """Create a sandbox environment and assign it to an agent.
 
     Lifecycle Management:
-    - Sandbox is acquired on first agent invocation for a thread (before_agent)
+    - With lazy_init=True (default): Sandbox is acquired on first tool call
+    - With lazy_init=False: Sandbox is acquired on first agent invocation (before_agent)
     - Sandbox is reused across multiple turns within the same thread
     - Sandbox is NOT released after each agent call to avoid wasteful recreation
     - Cleanup happens at application shutdown via SandboxProvider.shutdown()
@@ -27,6 +28,17 @@ class SandboxMiddleware(AgentMiddleware[SandboxMiddlewareState]):
 
     state_schema = SandboxMiddlewareState
 
+    def __init__(self, lazy_init: bool = True):
+        """Initialize sandbox middleware.
+
+        Args:
+            lazy_init: If True, defer sandbox acquisition until first tool call.
+                      If False, acquire sandbox eagerly in before_agent().
+                      Default is True for optimal performance.
+        """
+        super().__init__()
+        self._lazy_init = lazy_init
+
     def _acquire_sandbox(self, thread_id: str) -> str:
         provider = get_sandbox_provider()
         sandbox_id = provider.acquire(thread_id)
@@ -35,6 +47,11 @@ class SandboxMiddleware(AgentMiddleware[SandboxMiddlewareState]):
 
     @override
     def before_agent(self, state: SandboxMiddlewareState, runtime: Runtime) -> dict | None:
+        # Skip acquisition if lazy_init is enabled
+        if self._lazy_init:
+            return super().before_agent(state, runtime)
+
+        # Eager initialization (original behavior)
         if "sandbox" not in state or state["sandbox"] is None:
             thread_id = runtime.context["thread_id"]
             print(f"Thread ID: {thread_id}")