fix: salvage gateway dedup and executor cleanup from PR #993

Salvages the two still-relevant fixes from PR #993 onto current main:
- use a 3-tuple LOCAL delivery key so explicit/local-origin targets are not duplicated
- shut down the previous agent-loop ThreadPoolExecutor when resizing the global pool

Adds regression tests for both behaviors.

gateway/delivery.py

@@ -161,7 +161,7 @@ class DeliveryRouter:
         
         # Always include local if configured
         if self.config.always_log_local:
-            local_key = (Platform.LOCAL, None)
+            local_key = (Platform.LOCAL, None, None)
             if local_key not in seen_platforms:
                 targets.append(DeliveryTarget(platform=Platform.LOCAL))