ci: automate GitHub releases

[somethingwithproof]

Summary

• create GitHub releases automatically for future rrd-* tag pushes
• support manually creating a release for an existing tag, including rrd-2.0.4
• validate tag names and safely no-op when a release already exists
• use the built-in GITHUB_TOKEN and gh CLI, with no third-party release action

Validation

• actionlint .github/workflows/release.yml
• git diff --check

After merge

Run GitHub Release manually with tag set to rrd-2.0.4 to backfill the GitHub release for the already-published PECL version.