Add Plan JSON view to run detail page

Shows collapsible JSON of execution plan below steps list.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/templates/runs/detail.html

@@ -94,6 +94,16 @@
             </div>
             {% endfor %}
         </div>
+
+        <!-- Plan JSON -->
+        <details class="mt-6">
+            <summary class="cursor-pointer text-gray-400 hover:text-white text-sm mb-2">
+                Show Plan JSON
+            </summary>
+            <div class="bg-gray-900 rounded-lg border border-gray-700 p-4 overflow-x-auto">
+                <pre class="text-sm text-gray-300 whitespace-pre-wrap">{{ plan | tojson(indent=2) }}</pre>
+            </div>
+        </details>
         {% else %}
         <p class="text-gray-500">No plan available for this run.</p>
         {% endif %}