When a release is created or updated with attachments, automatically
compute SHA256 checksums for every file and attach a checksums.sha256
manifest file. The manifest follows the standard sha256sum format:
<hash> <filename>
Existing checksums.sha256 files are replaced when attachments change.
Checksums are generated for both CreateRelease and UpdateRelease flows.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>