feat(KB-016): add task duplicate command

- Add duplicateTask() method to core store with fresh state reset
- Add CLI command handler and register duplicate subcommand
- Add POST /tasks/:id/duplicate API endpoint to dashboard
- Add comprehensive tests for store, CLI, and API routes
- Add pi extension tool for task duplication
- Add changeset for minor release bump

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

@@ -80,6 +80,7 @@ describe("kb pi extension", () => {
         "kb_task_attach",
         "kb_task_pause",
         "kb_task_unpause",
+        "kb_task_duplicate",
         "kb_task_import_github",
         "kb_task_import_github_issue",
         "kb_task_browse_github_issues",