Register partner_run_gate_enabled feature flag (#16012)

The frontend's partner-nodes run gate (Comfy-Org/ComfyUI_frontend#15248)
reads this key from /features as its kill switch. Registering it makes
--feature-flag partner_run_gate_enabled=false coerce to a real boolean;
unregistered keys pass through as strings, and the string "false" is
truthy to the frontend, so the switch would silently not work.

comfy_api/feature_flags.py

@@ -30,6 +30,11 @@ CLI_FEATURE_FLAG_REGISTRY: dict[str, FeatureFlagInfo] = {
         "default": False,
         "description": "Signal the frontend that telemetry collection is enabled",
     },
+    "partner_run_gate_enabled": {
+        "type": "bool",
+        "default": True,
+        "description": "Gate the local Run button behind sign-in when the graph contains partner nodes",
+    },
 }