fix(stabilization): harden attachment lifecycle and agent guard signals (#5420)

* fix: harden stabilization attachment and agent guards

* fix(uploads): preserve durable references during cleanup

* fix(uploads): close cleanup and compaction races

src/tool_utils.py

@@ -9,6 +9,7 @@ import json
 from src.constants import MAX_OUTPUT_CHARS
 
 _mcp_manager = None
+_upload_handler = None
 
 # ---------------------------------------------------------------------------
 # MCP Manager singleton
@@ -23,6 +24,21 @@ def get_mcp_manager():
     """Get the global MCP manager instance."""
     return _mcp_manager
 
+
+# ---------------------------------------------------------------------------
+# Shared upload lifecycle handler
+# ---------------------------------------------------------------------------
+
+def set_upload_handler(handler):
+    """Register the process's UploadHandler without importing app modules."""
+    global _upload_handler
+    _upload_handler = handler
+
+
+def get_upload_handler():
+    """Return the shared UploadHandler used by route and agent writers."""
+    return _upload_handler
+
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------