✨ feat: fold checklist validation into body action

[zrr1999]

Summary

• Fold checked PR checklist validation into zendev-validate-body behind a --require-checklist option.
• Add matching validate-body action inputs (require-checklist, checklist-section, and fail-on-empty-checklist) and remove the separate validate-checklist action path.
• Configure a hatch-vcs fallback version so bundled GitHub Action archive checkouts can build with uvx --from without consumer workflows setting SETUPTOOLS_SCM_* environment variables.

Validation

• prek run
• prek run --all-files
• uv run pytest -q
• uvx ruff check
• uv run pyright
• uvx ty check .
• Archive checkout smoke test: uvx --from /tmp/zendev-archive-sim2 zendev-validate-title "📝 docs: test"
• Archive checkout checklist smoke test: uvx --from /tmp/zendev-archive-sim2 zendev-validate-body ... --require-checklist
• git diff --cached --check

Notes

After this is released, downstream repos can use only actions/validate-body with require-checklist: "true"; they should not need zendev packaging-specific environment variables.