test: add module-aligned source coverage

[jasonsaayman]

Summary

• Split the catch-all test file into module-aligned tests for action, comparison, config, paths, report, and tarball behavior.
• Expanded edge-case coverage for path safety, tarball parsing/download failures, report writing, gzip comparison output, and action orchestration with a mocked axios npm tarball.
• Synced and archived the OpenSpec add-src-test-suite change, and strengthened repo guidance so code changes include tests by default.

Testing

• pnpm test

---

Summary by cubic

Reorganized the test suite into module-aligned files and added focused edge-case coverage to improve safety and maintainability. Added an OpenSpec “source-test-suite” requirement and repo guidance so future changes include tests by default.

• Refactors
  • Split the catch-all test into tests/action.test.js, comparison.test.js, config.test.js, paths.test.js, report.test.js, and tarball.test.js; removed tests/index.test.js.
  • Added an orchestration test for run() that mocks an npm axios tarball download and verifies GITHUB_OUTPUT and report output.
  • Expanded coverage for path normalization/safety, tarball parsing and failures, gzip size/delta math, and safe report writing.
  • Kept tests deterministic and offline; no new runtime or dev dependencies.
  • Synced OpenSpec: added source-test-suite specs, archived the change, and updated openspec/config.yaml and AGENTS.md to require module-aligned tests.
  • No runtime behavior changes; pnpm test compiles to lib/ before running.

^{Written for commit 0c1d537. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 15 files

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