🔄 Migrate to folke/github Reusable Workflows

This PR migrates our CI/CD workflows to use folke/github reusable workflows, standardizing our setup with other popular Neovim plugins.

Changes

Removed (280 lines)

• ❌ tests.yml → Replaced by folke CI
• ❌ commitlint.yml → Replaced by pr.yml
• ❌ release-please.yml → Now handled by folke CI

Added (122 lines)

• ✅ ci.yml → Calls folke's reusable CI workflow (tests, lint, format, release-please)
• ✅ pr.yml → PR title validation (conventional commits)
• ✅ post-release.yml → Creates rockspec PRs after releases
• ✅ scripts/test → Test script for CI auto-detection
• ✅ make check → Run all CI checks locally

Kept Unchanged

• ✅ release-to-luarocks.yml → LuaRocks publishing
• ✅ security-scan.yml → Security scanning

Benefits

✅ Simpler: 158 fewer lines of workflow code (-58%)
✅ Standardized: Uses folke's battle-tested CI patterns
✅ Maintained: CI updates happen upstream
✅ Complete: All features preserved (tests, lint, format, releases, rockspecs, LuaRocks)
✅ Testable: make check runs same checks as CI

What folke CI Provides

1. Stylua formatting checks
2. Tests via scripts/test
3. Debug detection (catches dd() calls)
4. Release automation via release-please
5. PR title validation (conventional commits)

Release Flow

Push to main
   ↓
folke CI: tests + lint + format
   ↓
folke CI: release-please (auto PR)
   ↓
Merge release PR → GitHub Release published
   ↓
post-release.yml: Create rockspec PR
   ↓
Auto-merge rockspec PR
   ↓
release-to-luarocks.yml: Publish to LuaRocks

Testing

All workflows have been validated and the test script works correctly:

./scripts/test  # ✅ All tests pass
make check      # ✅ Lint, format-check, test