feat: add workflow_dispatch trigger to release workflow

[tankdonut]

Summary

• Add workflow_dispatch trigger to enable manual release execution from the GitHub Actions UI
• Change diff base from HEAD~1 to the last release tag (git describe --tags --abbrev=0) so the workflow works correctly regardless of how it was triggered

Why

GitHub Actions does not trigger workflows from pushes made by github-actions[bot] (using GITHUB_TOKEN). This means if the bot updates metadata.yaml, the release workflow won't fire automatically. Adding workflow_dispatch allows manual re-triggering from the UI.

The diff base change ensures workflow_dispatch works even when the last commit didn't touch metadata.yaml — it compares against the last release tag instead.

Changes

Location	Change
on: section	Added workflow_dispatch trigger
"Store metadata diff" step	Resolve diff base to last tag, fall back to HEAD~1, expose as DIFF_BASE env var
"Calculate CalVer tag" step	Use $DIFF_BASE env var instead of hardcoded HEAD~1
"Update CHANGELOG.md" step	Use $DIFF_BASE env var instead of hardcoded HEAD~1

Testing

• All 146 tests pass
• Ruff lint clean
• YAML syntax validated