fix(dag): add defensive check for ProcessManager singleton

Add runtime check to provide better error message if ProcessManager()
returns None. This should never happen with proper singleton pattern,
but helps diagnose issues in edge cases.

pkg/hanzo-mcp/hanzo_mcp/tools/shell/dag_tool.py

@@ -206,6 +206,10 @@ backgrounded. Use ps tool to monitor: ps --logs <id>, ps --kill <id>"""
         start_time = datetime.now()
         node_id = f"shell_{id(cmd)}"
         process_manager = ProcessManager()
+        
+        # Defensive check - should never happen but provides better error message
+        if process_manager is None:
+            raise RuntimeError("ProcessManager() returned None - singleton initialization failed")
 
         try:
             # Merge environment

pkg/hanzo-tools-shell/hanzo_tools/shell/dag_tool.py

@@ -178,6 +178,10 @@ backgrounded. Use ps tool to monitor: ps --logs <id>, ps --kill <id>"""
         start_time = datetime.now()
         node_id = f"shell_{id(cmd)}"
         process_manager = ProcessManager()
+        
+        # Defensive check - should never happen but provides better error message
+        if process_manager is None:
+            raise RuntimeError("ProcessManager() returned None - singleton initialization failed")
 
         try:
             run_env = os.environ.copy()