feat: US-006 - Add AsyncCallback type and AsyncTool interface

- Define AsyncCallback function type for async tool completion notification
- Define AsyncTool interface with SetCallback method
- Add comprehensive godoc comments with usage examples
- This enables tools like SpawnTool to notify completion asynchronously

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.ralph/prd.json

@@ -91,7 +91,7 @@
         "Typecheck passes"
       ],
       "priority": 6,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {