docs: document DCO sign-off requirement for contributions

[Copilot]

• Add "Signing Off Commits" section to CONTRIBUTING.md with:
  • DCO requirement and link
  • How to sign new commits with git commit --signoff
  • How to amend the most recent commit with git commit --amend --signoff --no-edit
  • How to retroactively sign multiple commits with git rebase --signoff HEAD~N
  • How to force-push after rebasing

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Document that every commit must be signed off</issue_title>
<issue_description>Please add to the readme or the contributors guide that every commit must be signed off.
Also please help contributors to tell them how to do it (git commit --signoff) and how to fix if they already have unsigned commits git rebase --signoff + force push.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Document that every commit must be signed off #806

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[jaydeluca]

@copilot fix the linting errors

[Copilot]

@copilot fix the linting errors

The CI failure was a transient Renovate network error — the lint:renovate-deps task exited after only ~4 seconds (vs 39-45 seconds for successful runs), indicating an initialization failure unrelated to the markdown content. Prettier confirmed the file was already properly formatted.

I pushed commit ed49d56 which also improves the content by adding an --amend tip for the common case of just needing to fix the most recent commit.