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.

tests/gateway/test_delivery.py

@@ -1,7 +1,7 @@
 """Tests for the delivery routing module."""
 
 from gateway.config import Platform, GatewayConfig, PlatformConfig, HomeChannel
-from gateway.delivery import DeliveryTarget, parse_deliver_spec
+from gateway.delivery import DeliveryRouter, DeliveryTarget, parse_deliver_spec
 from gateway.session import SessionSource
 
 
@@ -85,3 +85,12 @@ class TestTargetToStringRoundtrip:
         reparsed = DeliveryTarget.parse(s)
         assert reparsed.platform == Platform.TELEGRAM
         assert reparsed.chat_id == "999"
+
+
+class TestDeliveryRouter:
+    def test_resolve_targets_does_not_duplicate_local_when_explicit(self):
+        router = DeliveryRouter(GatewayConfig(always_log_local=True))
+
+        targets = router.resolve_targets(["local"])
+
+        assert [target.platform for target in targets] == [Platform.LOCAL]