Use native S-expression for recipe/plan display

- Display recipe's original S-expression when available (code is data)
- Fall back to generating S-expression from plan for legacy JSON
- Run service now prefers .sexp plan files over .json
- Add get_run_plan_sexp() for direct S-expression access

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

app/services/run_service.py

@@ -482,10 +482,28 @@ class RunService:
 
     async def get_run_plan(self, run_id: str) -> Optional[Dict[str, Any]]:
         """Get execution plan for a run."""
-        plan_path = self.cache_dir / "plans" / f"{run_id}.json"
-        if plan_path.exists():
-            with open(plan_path) as f:
-                return json.load(f)
+        # Prefer S-expression plan
+        sexp_path = self.cache_dir / "plans" / f"{run_id}.sexp"
+        if sexp_path.exists():
+            with open(sexp_path) as f:
+                return {"sexp": f.read(), "format": "sexp"}
+
+        # Fall back to JSON for legacy plans
+        json_path = self.cache_dir / "plans" / f"{run_id}.json"
+        if json_path.exists():
+            with open(json_path) as f:
+                plan = json.load(f)
+                plan["format"] = "json"
+                return plan
+
+        return None
+
+    async def get_run_plan_sexp(self, run_id: str) -> Optional[str]:
+        """Get execution plan as S-expression string."""
+        sexp_path = self.cache_dir / "plans" / f"{run_id}.sexp"
+        if sexp_path.exists():
+            with open(sexp_path) as f:
+                return f.read()
         return None
 
     async def get_run_artifacts(self, run_id: str) -> List[Dict[str, Any]]: