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

src/agent.rs

@@ -1124,7 +1124,7 @@ async fn process_feedback(
                 }).collect();
 
                 // Apply docker-cache diff
-                state.apply_plan_diff(new_steps);
+                let diff = state.apply_plan_diff(new_steps);
 
                 // Broadcast updated plan
                 let _ = broadcast_tx.send(WsMessage::PlanUpdate {
@@ -1134,6 +1134,10 @@ async fn process_feedback(
 
                 tracing::info!("[workflow {}] Plan revised via feedback. First actionable: {:?}",
                     workflow_id, state.first_actionable_step());
+
+                // Log the diff so frontend can show what changed
+                let diff_display = format!("```diff\n{}\n```", diff);
+                log_execution(pool, broadcast_tx, workflow_id, 0, "revise_plan", "计划变更", &diff_display, "done").await;
             }
         }
     } else {