feat: add release management workflow

[pchalamet]

Summary

• add changelog-driven release management similar to FScript
• add changelog validation and release helper scripts
• update GitHub workflows for branch, PR, tag, and release publishing flows
• replace RELEASE_NOTES.md with a reconstructed CHANGELOG.md

Validation

• make verify-changelog
• make release-prepare version=0.5.0-beta dryrun=true
• bash -n .github/scripts/*.sh

Notes

• local dotnet test could not be executed because global.json pins SDK 10.0.100 while this machine currently has 10.0.203 installed