[rocklambros]

What changed

The dco check is now required on integration, and many contributors have never signed off a commit. This adds a "How to sign your commits" section to CONTRIBUTING.md that assumes no prior knowledge, and points the check's failure message at it.

Which issue does this implement

Refs #52

• This is an editorial correction (typo, grammar, link, formatting) with no change in
  meaning, so it needs no issue

Base branch

• integration, because this touches the specification, schemas, a reference
  implementation, an adapter, tests, or CI
• main, because every changed path is on the documentation lane allowlist in
  CONTRIBUTING.md

Type of change

• Specification change (schema, hooks, events, AgBOM)
• Reference implementation or adapter
• Documentation
• Tooling or CI
• Governance (licensing, security policy, contributor docs)

I tested this

• I synced my branch with the base branch before opening this
• uv run pytest -v passes on my machine
• uv run mkdocs build --strict passes on my machine

The rebase steps ran on copies of PR #223, PR #246, and PR #297 with no conflicts and left no unsigned commit. A separate scratch repository confirmed that a sign-off survives a conflict and git rebase --continue, and that --signoff works on a branch already at the tip.

Checklist

• Commits are signed off with git commit -s (required by the DCO)
• Prose follows STYLE.md
• No secrets, tokens, or internal URLs in the diff

Security

• This change has no security impact