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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.github/CONTRIBUTING.md (1)

122-137: LGTM!

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📝 Summary

Summary by CodeRabbit

• Documentation
  • Added contribution guidance requiring signed commits on changes merged into the default branch, including requirements for SSH signing and account setup.
  • Clarified that apps, bots and workflows should create commits through GitHub’s API or the estate action.
  • Documented that pull requests must be squash-merged, rebase merges are unavailable, and branches with unsigned commits must be recreated using signed commits.

Walkthrough

The contribution guide adds signed-commit requirements for commits reaching the default branch. It describes signing methods for people and automated actors, and states how to handle unsigned branches and merge pull requests.

Changes

Signed commit policy

Layer / File(s)	Summary
Signing and merge procedures .github/CONTRIBUTING.md	The guide requires registered SSH signing keys and verified committer email for people and interactive agents. It directs apps, bots and workflows to use GitHub’s API or the estate signed-push action. It also specifies squash merges and requires unsigned branches to be recreated with signed commits.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~8 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to 9c2f3

This documentation-only change adds signed-commit guidance. No actionable merge risk is established by the available evidence.

Architecture Summary

Architecture risk: 🔵 Low · up to 9c2f3

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 signed-commit requirements and procedures: people and interactive agents sign with a registered SSH signing key and verified committer email; apps, bots and workflows use the API or estate signed-push action; PRs use squash merges because every branch commit is checked, and unsigned commits require recreating the branch with signed commits. Rebase-merge is stated to be disabled.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a Signed commits section to CONTRIBUTING.
Description check	✅ Passed	The description directly explains the documentation change, its policy basis, and the signing requirements.
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 signs each commit with care,
With SSH keys ready there.
Squash the branch, keep records neat,
Recreate unsigned commits complete.
The guide now shows the way.}

_{Comment @coderabbitai help to get the list of available commands.}