Fix automatic GitHub Packages release versioning

[marcialwushu]

Summary

• replace manual release inputs with an input-free publish-packages workflow
• calculate SemVer from the latest reachable release tag and repository diff
• publish PromptKit before Prompts with one shared version
• create the release tag only after both GitHub Packages publications succeed
• reject releases from branches other than main

Validation

• actionlint .github/workflows/*.yml
• 58 unit and contract tests passed
• first-release plan simulation: base root commit, impact MAJOR, version 0.1.0 from version.json

Failure fixed

The previous manual run allowed base-ref to point to the current commit and accepted identical previous/next version inputs. The new orchestration exposes no manual version fields.