chore: 100 stars is inclusive and add quality-check workflow

Two related follow-ups to PR #3306:

1. **Star rule is inclusive**: change "More than 100 stargazers" to
   "At least 100 stargazers" in CONTRIBUTING.md and the PR template.
   Repos with exactly 100 stars qualify — the bar is 100 flat, not 101.
   Hidden-gem range narrowed to 75-99 accordingly.

2. **Add the PR quality-check workflow** that was drafted in #3307 but
   did not land in master. Runs on every PR touching README.md,
   extracts new github.com URLs, and checks each one against the
   automatable hard rules (stars, contributors, archived, license,
   with stale-commit shown as a soft warning).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>