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.

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

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Reviewing files that changed from the base of the PR and between 6f2a119 and e195c67.

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• .github/CONTRIBUTING.md

📝 Summary

Summary by CodeRabbit

• Documentation
  • Added guidance on commit signing requirements for changes reaching the default branch, including supported signing methods and merge workflows.

Walkthrough

The contribution guide now documents signed-commit requirements, permitted write paths for apps, bots and workflows, and squash-merge rules. It states that rebase-merge is disabled.

Changes

Signed commit guidance

Layer / File(s)	Summary
Document signed commit rules .github/CONTRIBUTING.md	The guide requires people and interactive agents to use a registered SSH signing key and verified committer email. It specifies write paths for apps, bots and workflows, requires squash merges, and states that rebase-merge is disabled.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~4 minutes

Change: Other

Merge Risk: 🔵 Low · up to 6f2a1

Contributors may configure the wrong signing method and fail to meet the stated commit policy. Align the earlier instructions with the SSH requirements before merging.

Architecture Summary

Architecture risk: 🔵 Low · up to 6f2a1

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 distinguishes signing methods: people and interactive agents use a registered SSH signing key with a verified committer email, while apps, bots and workflows write through specified API or merge paths rather than pushing local commits. It also specifies squash merges and states that 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 signed-commit policy content.
Title check	✅ Passed	The title clearly and concisely identifies the addition of a Signed commits section 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 signs before the hop
With keys registered, ears on top
Bots take paths the guide makes clear
Squash each branch, then disappear
No unsigned push shall cross the gate
The rabbit thumps: commits are straight}

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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:
- Around line 121-123: Update the local setup and commit-style guidance to match
the signing policy: require a GitHub-registered SSH signing key for people and
interactive agents, and require signed commits without claiming all commits use
GPG. Do not require developers to configure a GPG key; keep GPG signing
applicable to unattended apps, bots, and workflows using the API.

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• .github/CONTRIBUTING.md

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