Split test suites for explicit runtime execution.
- Move model-related tests under `src/models/*`.
- Move CLI/integration tests under `src/integration/*`.
- Add `src/store.helpers.unit.test.ts` for helper unit coverage.
- Add shared Vitest config with default timeout and suite organization.
- Remove legacy flat test files from `src/` root.
- Keep core test commands in scripts supporting unit/models/integration runs.