Backfill existing releases to mod platforms

[xefensor]

Summary

• add a manual workflow for publishing an existing GitHub release to Modrinth, CurseForge, or both
• download the exact existing GitHub release JAR instead of rebuilding current source
• verify the SHA-256 asset when present
• use release notes from the matching tagged changelog
• keep Modrinth and CurseForge as independent steps so a failed destination can be retried without duplicating the successful one
• document the one-time backfill and the normal future release process

Intended first run

• version: 0.2.1
• Modrinth: enabled
• CurseForge: enabled
• GitHub: untouched

Safety

The workflow is manual-only. It requires an existing matching tag and GitHub release, does not receive GitHub write permission, and cannot create or modify a GitHub release.