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.

environments/agent_loop.py

@@ -39,7 +39,9 @@ def resize_tool_pool(max_workers: int):
     Safe to call before any tasks are submitted.
     """
     global _tool_executor
+    old_executor = _tool_executor
     _tool_executor = concurrent.futures.ThreadPoolExecutor(max_workers=max_workers)
+    old_executor.shutdown(wait=False)
     logger.info("Tool thread pool resized to %d workers", max_workers)
 
 logger = logging.getLogger(__name__)