Store provenance on IPFS instead of local files

- Add add_json() to ipfs_client for storing JSON data
- Update render_effect task to store provenance on IPFS
- Update execute_dag task to store DAG provenance on IPFS
- Add provenance_cid field to RunStatus model
- Extract provenance_cid from task results

Provenance is now immutable and content-addressed, enabling:
- Cross-L2 verification
- Bitcoin timestamping for dispute resolution
- Complete audit trail on IPFS

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ipfs_client.py

@@ -102,6 +102,22 @@ def add_bytes(data: bytes, pin: bool = True) -> Optional[str]:
         return None
 
 
+def add_json(data: dict, pin: bool = True) -> Optional[str]:
+    """
+    Serialize dict to JSON and add to IPFS.
+
+    Args:
+        data: Dictionary to serialize and store
+        pin: Whether to pin the data (default: True)
+
+    Returns:
+        IPFS CID or None on failure
+    """
+    import json
+    json_bytes = json.dumps(data, indent=2, sort_keys=True).encode('utf-8')
+    return add_bytes(json_bytes, pin=pin)
+
+
 def get_file(cid: str, dest_path: Path) -> bool:
     """
     Retrieve a file from IPFS and save to destination.