feat: meta-PR — use gem-contribute against a real downstream

[cdhagmann]

The proof point for v1: use gem-contribute itself to open a real PR on a real downstream project that depends on it.

Scope:

• Find a real Ruby project that depends on gem-contribute (or pick a target downstream we want it adopted in)
• Use the fix flow end-to-end against a real issue
• Open the PR
• Document the experience as a release-day blog post or in the GitHub release notes

See: ROADMAP Phase 6