Save plans as S-expressions under content hash

- Orchestrator saves plan as {plan_id}.sexp (content-addressed)
- Also saves {run_id}.sexp for easy lookup by run
- Falls back to JSON for legacy plans without to_sexp_string()

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

tasks/orchestrate.py

@@ -296,11 +296,22 @@ def run_recipe(
 
     logger.info(f"Generated plan with {len(plan.steps)} steps")
 
-    # Save plan for debugging
+    # Save plan as S-expression (content-addressed by plan_id hash)
     PLAN_CACHE_DIR.mkdir(parents=True, exist_ok=True)
-    plan_path = PLAN_CACHE_DIR / f"{plan.plan_id}.json"
-    with open(plan_path, "w") as f:
-        f.write(plan.to_json())
+    plan_sexp_path = PLAN_CACHE_DIR / f"{plan.plan_id}.sexp"
+    if hasattr(plan, 'to_sexp_string'):
+        with open(plan_sexp_path, "w") as f:
+            f.write(plan.to_sexp_string())
+    else:
+        # Fallback to JSON for legacy plans
+        plan_json_path = PLAN_CACHE_DIR / f"{plan.plan_id}.json"
+        with open(plan_json_path, "w") as f:
+            f.write(plan.to_json())
+
+    # Also save a reference by run_id for easy lookup
+    run_plan_path = PLAN_CACHE_DIR / f"{run_id}.sexp"
+    if plan_sexp_path.exists():
+        run_plan_path.write_text(plan_sexp_path.read_text())
 
     # Phase 3: Execute
     logger.info("Phase 3: Executing plan...")