move release logic into go helper

[lancekrogers]

Summary

• keep just as the operator interface but move release behavior out of inline bash and into a Go helper
• add unit-tested version/tag selection and release guardrails under internal/releaseutil
• reduce .justfiles/release.just to thin entrypoints only

Validation

• go test ./internal/releaseutil/...
• just release
• just release current
• just release check
• just test unit

Why

Inline bash is a poor place to keep release orchestration. This refactor makes the release path testable, debuggable, and consistent with the repo being Go-first.