chore: raise inclusion bar with stricter contribution rules

Rewrites CONTRIBUTING.md and PULL_REQUEST_TEMPLATE.md to enforce a
clearer, stricter bar for inclusion in the list.

New hard rules (all required):
- More than 100 stars (was 50)
- Commit in last 24 months (was "recent", now explicit)
- More than one contributor (was "can be rejected", now required)
- Not archived
- OSI-approved open-source license
- Supports Swift Package Manager
- Targets iOS 14+ / tvOS 14+ / macOS 11+ / watchOS 7+
- Built with Swift 5.5+ (or Obj-C on Xcode 15+)
- English README with install + usage example
- Not a paid library, paid app, or paid course (free tier counts)

Changes from previous rules:
- Tests dropped from hard rule to soft signal
- Carthage no longer required; SPM is now the baseline
- iOS/Swift floors bumped to iOS 14 / Swift 5.5
- Added "hidden gem" exception for 75-100 star projects with justification
- Added explicit deletion criteria
- Added soft signals borrowed from awesome-go / awesome-python
  (SemVer tag, repo age, promotional language, category minimum)

PR template becomes an actionable checklist with an explicit warning
that unchecked boxes mean the PR will be closed.

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