Add a button to remove the plan UI (#52360)

## Context

This PR adds a button to remove the plan UI, if the user wants.

## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

crates/acp_thread/src/acp_thread.rs

@@ -2072,6 +2072,11 @@ impl AcpThread {
         cx.notify();
     }
 
+    pub fn clear_plan(&mut self, cx: &mut Context<Self>) {
+        self.plan.entries.clear();
+        cx.notify();
+    }
+
     #[cfg(any(test, feature = "test-support"))]
     pub fn send_raw(
         &mut self,

crates/agent_ui/src/conversation_view/thread_view.rs

@@ -2540,7 +2540,17 @@ impl ThreadView {
                 this.border_b_1().border_color(cx.theme().colors().border)
             })
             .child(Disclosure::new("plan_disclosure", plan_expanded))
-            .child(title)
+            .child(title.flex_1())
+            .child(
+                IconButton::new("dismiss-plan", IconName::Close)
+                    .icon_size(IconSize::XSmall)
+                    .shape(ui::IconButtonShape::Square)
+                    .tooltip(Tooltip::text("Clear plan"))
+                    .on_click(cx.listener(|this, _, _, cx| {
+                        this.thread.update(cx, |thread, cx| thread.clear_plan(cx));
+                        cx.stop_propagation();
+                    })),
+            )
             .on_click(cx.listener(|this, _, _, cx| {
                 this.plan_expanded = !this.plan_expanded;
                 cx.notify();