Remove 'Add Comment' from tree view actions

**Correct UX: Comments belong in diff view, not tree actions**

**Removed:** 💬 Add Comment button from tree view Actions section

**Rationale:**
- Comments should be triggered from diff view on specific lines
- Tree view actions are for overall review decisions
- Keeps tree view focused on high-level review actions

**Remaining tree view actions:**
- ✅ Request Changes
- 📝 Checkpoint Work
- ↩️ Close Review

**Next:** Implement inline commenting in diff view for line-specific feedback

Author: nikomatsakis

extension/src/syntheticPRTreeProvider.ts

@@ -177,12 +177,6 @@ export class SyntheticPRTreeProvider implements vscode.TreeDataProvider<PRTreeIt
         if (element.itemType === 'actions') {
             // Show feedback actions
             return Promise.resolve([
-                new PRTreeItem(
-                    '💬 Add Comment',
-                    vscode.TreeItemCollapsibleState.None,
-                    'action',
-                    { action: 'comment' }
-                ),
                 new PRTreeItem(
                     '✅ Request Changes',
                     vscode.TreeItemCollapsibleState.None,