docs: add Signed commits section to CONTRIBUTING

[hyperpolymath]

Adds a Signed commits section to this repo's CONTRIBUTING, per owner ruling D218. The estate policy is docs/SIGNING-POLICY.adoc in hyperpolymath/standards.

This repo's default branch is covered by the zero-bypass Require-Signed-Commits ruleset, and rebase-merge is off. The section tells contributors what that requires:

• People and interactive agents sign with an SSH signing key.
• Apps, bots and workflows write through the API, so GitHub signs their commits.
• PRs are merged with squash.

If the file already had its own signing section, that section is replaced in place instead of adding a second one. Lines elsewhere that told people to sign with GPG are changed to match the policy (SSH for people).

This is a docs-only change. The commit was created through createCommitOnBranch, so GitHub signs it.

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• .github/CONTRIBUTING.md

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📝 Summary

Summary by CodeRabbit

• Documentation
  • Documented the requirement for signed commits to reach the default branch, including requirements for people, automated tools and workflows.
  • Clarified that pull requests must be squash-merged and that rebase merging is unavailable.

Walkthrough

The contribution guide adds instructions for signing commits and merging pull requests. It describes signing requirements for people and automated commit creation, and states that pull requests must use squash merges.

Changes

Commit Signing and Merging

Layer / File(s)	Summary
Signing and merge requirements .github/CONTRIBUTING.md	The guide describes the default-branch ruleset, signing requirements, approved commit creation methods for apps and workflows, and the squash-merge requirement. It also states that rebase merging is disabled.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Other

Suggested reviewers: metadatastician

Merge Risk: 🔵 Low · up to 99441

Contributors who can update their existing branch can keep the same PR; others may need a replacement. The current guidance creates avoidable PR churn, a localized documentation issue.

Architecture Summary

Architecture risk: 🔵 Low · up to 99441

The changed surface does not map to a changed system, dependency edge, entrypoint, or external dependency.

Changed systems: None identified.

Architecture concerns
No architecture-level concerns identified.

Review details

Before / after behavior

• observed — Modified behavior in .github/CONTRIBUTING.md: Adds commit-signing and merge instructions: the default-branch ruleset rejects unsigned pushes; people and interactive agents use a registered SSH signing key and verified committer email; apps, bots and workflows create commits through the API or signed-push action; and PRs use squash merges because unsigned branch commits block merging, while rebase-merge is disabled.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the documentation change and its signing policy requirements.
Title check	✅ Passed	The title is concise, specific, and accurately summarises the addition of signed-commit guidance to CONTRIBUTING.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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_{A rabbit checks each signed commit,
Then finds the squash-merge path is fit.
SSH keys keep the signatures clear,
Bots use approved routes to appear.
The guide now tells the steps to take.}

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @.github/CONTRIBUTING.md:
- Line 137: Update the contribution guidance around `git cherry-pick -S` to
direct contributors to push signed commits to the existing PR’s head branch when
it can be updated, and create a new branch and PR only when it cannot.

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🔇 Additional comments (1)

.github/CONTRIBUTING.md (1)

122-136: LGTM!

Also applies to: 138-138