feat(chat): use LaunchDarkly flag for copilot SDK rollout

Replace static CHAT_USE_CLAUDE_AGENT_SDK env var with a LaunchDarkly
feature flag (copilot-sdk) for per-user rollout control. The env var
value serves as the default when LD is not configured or the flag
doesn't exist yet.

autogpt_platform/backend/backend/api/features/chat/routes.py

@@ -12,6 +12,7 @@ from fastapi.responses import StreamingResponse
 from pydantic import BaseModel
 
 from backend.util.exceptions import NotFoundError
+from backend.util.feature_flag import Flag, is_feature_enabled
 
 from . import service as chat_service
 from . import stream_registry
@@ -394,8 +395,12 @@ async def stream_chat_post(
                 },
             )
 
-            # Choose service based on configuration
-            use_sdk = config.use_claude_agent_sdk
+            # Choose service based on LaunchDarkly flag (falls back to config default)
+            use_sdk = await is_feature_enabled(
+                Flag.COPILOT_SDK,
+                user_id or "anonymous",
+                default=config.use_claude_agent_sdk,
+            )
             stream_fn = (
                 sdk_service.stream_chat_completion_sdk
                 if use_sdk

autogpt_platform/backend/backend/util/feature_flag.py

@@ -38,6 +38,7 @@ class Flag(str, Enum):
     AGENT_ACTIVITY = "agent-activity"
     ENABLE_PLATFORM_PAYMENT = "enable-platform-payment"
     CHAT = "chat"
+    COPILOT_SDK = "copilot-sdk"
 
 
 def is_configured() -> bool: