Add IPFS CID support for asset lookup

- Upload endpoint returns both CID and content_hash
- Cache manager handles both SHA3-256 hashes and IPFS CIDs
- get_by_cid() fetches from IPFS if not cached locally
- Execute tasks support :cid in addition to :hash

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

tasks/execute_sexp.py

@@ -197,13 +197,14 @@ def execute_step_sexp(
     try:
         # Handle SOURCE nodes
         if node_type == "SOURCE":
-            content_hash = config.get("hash")
-            if not content_hash:
-                raise ValueError("SOURCE step missing :hash")
+            # Support both :cid (new IPFS) and :hash (legacy)
+            content_id = config.get("cid") or config.get("hash")
+            if not content_id:
+                raise ValueError("SOURCE step missing :cid or :hash")
 
-            path = cache_mgr.get_by_content_hash(content_hash)
+            path = cache_mgr.get_by_content_hash(content_id)
             if not path:
-                raise ValueError(f"SOURCE input not found: {content_hash[:16]}...")
+                raise ValueError(f"SOURCE input not found: {content_id[:16]}...")
 
             output_path = str(path)
             complete_task(cache_id, worker_id, output_path)