feat: show plan diff in execution log when revise_plan is called

- apply_plan_diff now returns a YAML unified diff string
- Pure Rust LCS diff implementation (no external dependency)
- revise_plan logs the diff to execution log with ```diff fencing
- Frontend renders diff with green/red syntax highlighting

web/src/components/ExecutionSection.vue

@@ -131,6 +131,25 @@ function formatLatency(ms: number): string {
   return ms + 'ms'
 }
 
+function escapeHtml(s: string): string {
+  return s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;')
+}
+
+function isDiffOutput(output: string): boolean {
+  return output.startsWith('```diff\n')
+}
+
+function renderDiff(output: string): string {
+  // Strip ```diff fences
+  const inner = output.replace(/^```diff\n/, '').replace(/\n```$/, '')
+  return inner.split('\n').map(line => {
+    const esc = escapeHtml(line)
+    if (line.startsWith('+')) return `<span class="diff-add">${esc}</span>`
+    if (line.startsWith('-')) return `<span class="diff-del">${esc}</span>`
+    return esc
+  }).join('\n')
+}
+
 function parseToolCalls(json: string): { name: string; arguments_preview: string }[] {
   try {
     return JSON.parse(json)
@@ -275,7 +294,7 @@ watch(logItems, () => {
             <div v-if="item.entry.tool_input && item.entry.tool_name !== 'text_response'" class="exec-command">
               <code>{{ item.entry.tool_input }}</code>
             </div>
-            <pre v-if="item.entry.output">{{ item.entry.output }}</pre>
+            <pre v-if="item.entry.output" :class="{ 'diff-output': isDiffOutput(item.entry.output) }" v-html="isDiffOutput(item.entry.output) ? renderDiff(item.entry.output) : escapeHtml(item.entry.output)"></pre>
           </div>
         </div>
       </template>
@@ -665,4 +684,18 @@ watch(logItems, () => {
   font-family: 'JetBrains Mono', 'Fira Code', monospace;
   font-size: 10px;
 }
+
+pre.diff-output .diff-add {
+  color: #22c55e;
+  background: rgba(34, 197, 94, 0.1);
+  display: inline-block;
+  width: 100%;
+}
+
+pre.diff-output .diff-del {
+  color: #ef4444;
+  background: rgba(239, 68, 68, 0.1);
+  display: inline-block;
+  width: 100%;
+}
 </style>