Replace plan JSON with colored S-expression display

- Add plan_to_sexp() to convert plan to S-expression format
- Syntax highlighting for S-expressions:
  - Pink: special forms (plan, recipe, def, ->)
  - Blue: primitives (source, effect, sequence, etc.)
  - Purple: keywords (:input, :name, etc.)
  - Green: strings
  - Yellow: parentheses
  - Gray: comments

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

app/routers/runs.py

@@ -27,6 +27,59 @@ from ..services.run_service import RunService
 router = APIRouter()
 logger = logging.getLogger(__name__)
 
+
+def plan_to_sexp(plan: dict, recipe_name: str = None) -> str:
+    """Convert a plan to S-expression format for display."""
+    if not plan or not plan.get("steps"):
+        return ";; No plan available"
+
+    lines = []
+    lines.append(f'(plan "{recipe_name or "unknown"}"')
+
+    # Group nodes by type for cleaner output
+    steps = plan.get("steps", [])
+
+    for step in steps:
+        step_id = step.get("id", "?")
+        step_type = step.get("type", "EFFECT")
+        inputs = step.get("inputs", [])
+        config = step.get("config", {})
+
+        # Build the step S-expression
+        if step_type == "SOURCE":
+            if config.get("input"):
+                # Variable input
+                input_name = config.get("name", config.get("input", "input"))
+                lines.append(f'  (source :input "{input_name}")')
+            elif config.get("asset"):
+                # Fixed asset
+                lines.append(f'  (source {config.get("asset", step_id)})')
+            else:
+                lines.append(f'  (source {step_id})')
+        elif step_type == "EFFECT":
+            effect_name = config.get("effect", step_id)
+            if inputs:
+                inp_str = " ".join(inputs)
+                lines.append(f'  (-> {inp_str} (effect {effect_name}))')
+            else:
+                lines.append(f'  (effect {effect_name})')
+        elif step_type == "SEQUENCE":
+            if inputs:
+                inp_str = " ".join(inputs)
+                lines.append(f'  (sequence {inp_str})')
+            else:
+                lines.append(f'  (sequence)')
+        else:
+            # Generic node
+            if inputs:
+                inp_str = " ".join(inputs)
+                lines.append(f'  ({step_type.lower()} {inp_str})')
+            else:
+                lines.append(f'  ({step_type.lower()} {step_id})')
+
+    lines.append(')')
+    return "\n".join(lines)
+
 RUNS_KEY_PREFIX = "artdag:run:"
 
 
@@ -258,6 +311,9 @@ async def get_run(
                             }
                         })
 
+        # Generate S-expression representation of the plan
+        plan_sexp = plan_to_sexp(plan, run.get("recipe_name"))
+
         templates = get_templates(request)
         return render(templates, "runs/detail.html", request,
             run=run,
@@ -266,6 +322,7 @@ async def get_run(
             run_inputs=run_inputs,
             dag_elements=dag_elements,
             output_media_type=output_media_type,
+            plan_sexp=plan_sexp,
             active_tab="runs",
         )