Make IPFS upload failures fatal - no local hash fallback

IPFS CIDs are the primary identifiers. If IPFS upload fails,
the operation must fail rather than silently using local hashes.

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

cache_manager.py

@@ -286,9 +286,7 @@ class L1CacheManager:
         # Upload to IPFS first to get the CID (primary identifier)
         cid = ipfs_client.add_file(source_path)
         if not cid:
-            # Fallback to local hash if IPFS unavailable
-            cid = file_hash(source_path)
-            logger.warning(f"IPFS unavailable, using local hash: {cid[:16]}...")
+            raise RuntimeError(f"IPFS upload failed for {source_path}. IPFS is required.")
 
         # Use CID as node_id if not provided
         if node_id is None: