Refactor to S-expression based execution with code-addressed cache IDs

Major changes:
- Add execute_recipe task that uses S-expression planner
- Recipe S-expression unfolds into plan S-expression with code-addressed cache IDs
- Cache IDs computed from Merkle tree of plan structure (before execution)
- Add ipfs_client.add_string() for storing S-expression plans
- Update run_service.create_run() to use execute_recipe when recipe_sexp available
- Add _sexp_to_steps() to parse S-expression plans for UI visualization
- Plan endpoint now returns both sexp content and parsed steps

The code-addressed hashing means each plan step's cache_id is:
  sha3_256({node_type, config, sorted(input_cache_ids)})

This creates deterministic "buckets" for computation results computed
entirely from the plan structure, enabling automatic cache reuse.

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

ipfs_client.py

@@ -118,6 +118,20 @@ def add_json(data: dict, pin: bool = True) -> Optional[str]:
     return add_bytes(json_bytes, pin=pin)
 
 
+def add_string(content: str, pin: bool = True) -> Optional[str]:
+    """
+    Add a string to IPFS and optionally pin it.
+
+    Args:
+        content: String content to add (e.g., S-expression)
+        pin: Whether to pin the data (default: True)
+
+    Returns:
+        IPFS CID or None on failure
+    """
+    return add_bytes(content.encode('utf-8'), pin=pin)
+
+
 def get_file(cid: str, dest_path: Path) -> bool:
     """
     Retrieve a file from IPFS and save to destination.