test: add regression coverage and CI

[adity982]

What changed

• make the insight engine side-effect free by selecting the most-starred repository without sorting caller-owned input
• add deterministic Node test-runner coverage for metrics, empty inputs, recent activity, scoring, and input-order stability
• add syntax checks and GitHub Actions coverage on Node.js 18, 20, and 22
• rewrite the README around the user problem, quickstart, API, validation, deployment, and operational limits
• add focused contributor guidance, issue/PR templates, and a dated shipping log

Why

The repository described itself as a reference implementation without automated tests or CI. Its most-starred calculation also reordered the repository array in place, which could surprise later consumers. This pass turns the core insight logic into a reviewable, regression-protected public project and improves the first-run experience for users and contributors.

Validation

• GitHub Actions run 30071220075 passed on Node.js 18, 20, and 22
• every matrix job passed dependency installation, JavaScript syntax checks, and regression tests
• compared the branch against main; it is ahead with no divergence
• reviewed the complete changed-file set through the GitHub compare API

Safety

No tokens, private profile exports, database credentials, or real user fixtures are included.