Clear tree view after user action

**Clean UX: Tree view clears after feedback action**

**Changes:**
- Added clearPR() method to SyntheticPRProvider
- Call clearPR() after user selects any action button
- Tree view shows 'No active pull request' after action

**Flow:**
1. User clicks action button (Request Changes, Checkpoint, Close Review)
2. Action processed and feedback sent to LLM
3. Tree view immediately clears to show completion
4. User knows their action was processed

**Benefits:**
- **Clear visual feedback** - User sees action was processed
- **Clean state** - No stale review UI after completion
- **Intuitive UX** - Tree view state matches review state

Author: nikomatsakis

extension/src/extension.ts

@@ -530,6 +530,8 @@ class DaemonClient implements vscode.Disposable {
             });
         }
 
+        // Clear tree view after action
+        this.syntheticPRProvider.clearPR();
         this.pendingFeedbackResolvers.delete(reviewId);
     }
 

extension/src/syntheticPRProvider.ts

@@ -457,6 +457,14 @@ export class SyntheticPRProvider implements vscode.Disposable {
         reply.thread.canReply = true;
     }
 
+    /**
+     * Clear the current PR from tree view
+     */
+    clearPR(): void {
+        this.treeProvider.clearPR();
+        this.currentPR = null;
+    }
+
     dispose(): void {
         this.commentController.dispose();
         this.treeProvider.clearPR();