feat(KB-198): add task delete CLI command with --force flag

- Add `kb task delete` CLI command with required --force flag for safety
- Implement runTaskDelete function with task existence validation
- Register kb_task_delete tool in Pi extension for chat agent access
- Add comprehensive unit tests for runTaskDelete and extension registration
- Wire delete command in CLI bin.ts with proper argument parsing

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

@@ -87,6 +87,7 @@ describe("kb pi extension", () => {
         "kb_task_browse_github_issues",
         "kb_task_archive",
         "kb_task_unarchive",
+        "kb_task_delete",
         "kb_task_plan",
       ];