feat(KB-034): add task archive/unarchive functionality

- Add 'archived' column to task store with archiveTask and unarchiveTask methods
- Add CLI commands: kb task archive <id> and kb task unarchive <id>
- Add pi extension tools for archive and unarchive operations
- Add dashboard API endpoints POST /api/tasks/:id/archive and /unarchive
- Add Archived column to board UI with archive/unarchive buttons
- Prevent drag-drop into archived column, add visual distinction
- Include duplicateTask from concurrent branch in merge resolution

packages/cli/src/__tests__/extension.test.ts

@@ -84,6 +84,8 @@ describe("kb pi extension", () => {
         "kb_task_import_github",
         "kb_task_import_github_issue",
         "kb_task_browse_github_issues",
+        "kb_task_archive",
+        "kb_task_unarchive",
       ];
 
       for (const name of expected) {