docs(standard): let a repo with no runtime dependencies omit the floors job

[lesnik512]

Summary

CI6 requires a floors job that resolves every direct dependency at its declared floor. The conformance audit flagged modern-di for lacking one, but modern-di declares no runtime dependencies and no extras, so the set the job would resolve is empty.

Adding the job anyway is worse than omitting it: with nothing to resolve low, it passes vacuously, and the only thing --resolution lowest-direct would touch is the unfloored dev group, which resolves to each tool's first-ever release and fails for reasons unrelated to what the package ships.

This is a clarification, not an exemption: the repo does not depart from the requirement, the requirement has nothing to test.

Changes

• CI6: one added permission, "A repo with no runtime dependencies and no extras has no floors to test and MAY omit the job (modern-di)."

Checklist

• Tests pass (uv run pytest: 132 passed, 29 skipped)
• mkdocs build --strict passes
• Lint, type check, uv build, metadata: not applicable, docs only