Fix #324: add CLI integration test suite

[roblevintennis]

• Add Vitest as devDependency; update npm test to run vitest run
• Add vitest.config.ts with 30s timeout and test/ glob
• Add test/helpers.ts: createTempDir, createMockTarball, createMockReferenceLibrary, createInitializedProject, MOCK_MANIFEST
• Add test/init.test.ts: 6 tests covering config creation, CSS tokens, error paths, --force re-init
• Add test/add.test.ts: 6 tests covering happy path, config recording, error paths
• Add test/list.test.ts: 3 tests covering component listing and error paths
• Add test/context.test.ts: 7 tests covering all output formats, splice-on-rerun, error paths
• Add test/playbook.test.ts: 7 tests with mocked fetch for --list, install, and error paths
• Add .github/workflows/cli-tests.yml: runs on push/PR to v2/cli/**
• Use process.chdir() for test isolation; rely on Vitest's process.exit interception