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>

app/routers/recipes.py

@@ -569,6 +569,7 @@ async def run_recipe(
             actor_id=ctx.actor_id,
             l2_server=ctx.l2_server,
             recipe_name=recipe.get("name"),  # Store name for display
+            recipe_sexp=recipe.get("sexp"),  # S-expression for code-addressed execution
         )
 
         if error: