Simplify `CONTRIBUTING.md` to standard PR workflows

[4444J99]

Participants felt the contribution guidelines were too strict or used non-standard terminology. We should align it with standard open-source workflows.

[4444J99]

Engagement disposition: distilled: CONTRIBUTING.md. I verified main has a simplified standard contribution guide: ways to contribute, pick an issue, fork/branch/change/test/open PR, style guidelines, and help path. QUICK_START.md also links first contributors to good-first-issue work, so PR #37 is stale-base superseded unless a distinct policy gap remains.