Document signed and signed-off contributions

[kwisatzh]

Summary

• Explain verified signatures and author-matching DCO sign-offs, and why both are required.
• Document SSH key setup, GitHub registration, and git commit -s -S.
• Provide tested repair commands, including forced rebase for already-up-to-date branches.
• Link each README to the contributor guide.

Validation

• Documentation checks and shell syntax pass.
• Real Git fixture verifies signed rebasing and matching trailers on two existing commits, plus latest-commit amendment.
• All 64 local DCO tests pass.

Documentation only; no application or workflow changes.