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/templates/runs/detail.html

@@ -156,15 +156,17 @@
             {% endfor %}
         </div>
 
-        <!-- Plan S-expression -->
+        <!-- Recipe/Plan S-expression -->
+        {% if plan_sexp %}
         <details class="mt-6" open>
             <summary class="cursor-pointer text-gray-400 hover:text-white text-sm mb-2">
-                Plan (S-expression)
+                Recipe (S-expression)
             </summary>
             <div class="bg-gray-900 rounded-lg border border-gray-700 p-4 overflow-x-auto">
                 <pre class="text-sm font-mono sexp-code">{{ plan_sexp }}</pre>
             </div>
         </details>
+        {% endif %}
 
         <style>
         .sexp-code {