Remove YAML support - S-expressions only

- Recipe service now only handles S-expressions
- Removed yaml import and all YAML parsing code
- Plans are just node outputs - cached by content hash
- Run service looks up plans from cache, falls back to legacy dir

Code is data. Everything is S-expressions.

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

tasks/orchestrate.py

@@ -309,14 +309,10 @@ def run_recipe(
         tmp_path = Path(tmp.name)
 
     # Store in cache (content-addressed, auto-pins to IPFS)
+    # Plan is just another node output - no special treatment needed
     cached, plan_ipfs_cid = cache_mgr.put(tmp_path, node_type="plan", move=True)
     logger.info(f"Plan cached: hash={cached.content_hash}, ipfs={plan_ipfs_cid}")
 
-    # Also save to plans dir for legacy lookup by run_id
-    PLAN_CACHE_DIR.mkdir(parents=True, exist_ok=True)
-    run_plan_path = PLAN_CACHE_DIR / f"{run_id}.sexp"
-    run_plan_path.write_text(plan_sexp)
-
     # Phase 3: Execute
     logger.info("Phase 3: Executing plan...")