docs: rewrite the contributor guide around repo-specific rules

[cplieger]

Summary

• Rewrites CONTRIBUTING.md to hold only what is specific to this repository. Commits, releases, synced files, checks, review and scope come from the shared contributing rules, which this guide now links instead of restating.
• Every rule that stays names what breaks when a contributor ignores it.
• Points the README's Contributing section at the guide.

Testing

• Two independent reviews checked each kept statement against the code and checked that nothing a contributor needs was dropped. A final check reran the claim checks after the fixes.
• A blind contributor read planned three typical changes using only the guide and the code, and reported anything the guide failed to tell it.
• markdownlint, link checks and the docs scanner pass.