fix(notion-writer): add "object": "block" to create_callout_block

Code review caught that all 9 other create_*_block factories include
"object": "block" as the first key, but the new callout factory was
missing it. The Notion API accepts both forms, but the inconsistency
would compound; align with the existing convention.

22/22 tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

custom-skills/32-notion-writer/code/scripts/notion_writer.py

@@ -318,6 +318,7 @@ def create_callout_block(text: str, alert_type: str) -> Dict[str, Any]:
     """Create a callout block with icon + color from a GitHub alert type."""
     icon, color = ALERT_TYPES[alert_type]
     return {
+        "object": "block",
         "type": "callout",
         "callout": {
             "rich_text": parse_rich_text(text),