Remove CLI integration tests and related configurations

- Deleted test helper file `test-helpers.ts` which contained utility functions for integration tests.
- Removed integration tests for `claude-code`, `codex`, `opencode`, `trae`, and `variable-interpolation` plugins.
- Eliminated TypeScript configuration files `tsconfig.json` and `tsconfig.test.json`.
- Removed Vitest configuration file `vitest.config.ts`.

Author: TrueNine