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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📝 Summary

Summary by CodeRabbit

• Documentation
  • Added guidance on commit-signing requirements and accepted signing methods.
  • Clarified that unsigned commits on pull request branches block merging, with instructions for recreating a signed branch.
  • Documented that squash merges are required and rebase merges are unavailable.

Walkthrough

The contribution guide adds instructions for signed commits. It covers signing methods for people and automated actors, and explains how unsigned commits affect pull request branches and merging.

Changes

Signed Commit Guidance

Layer / File(s)	Summary
Commit signing and merge rules .github/CONTRIBUTING.md	Adds signing requirements and workflows for people and automated actors. It also describes rules for unsigned commits, squash merges, and recreating a branch with signed commits.

Priority: ⬇️ Low

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

Change: Other

Merge Risk: 🔵 Low · up to 91b99

Contributors can update the existing PR after re-signing; the new-PR instruction may unnecessarily split review discussion. This is a small documentation correction, not a merge blocker.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 91b99

The change documents signing and merge requirements without changing enforcement. Actual branch protections and merge settings remain unverified; no introduced security weakness has been established.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The unresolved question concerns this repository's default-branch commit acceptance and merge controls. The available evidence does not establish a bypass, gained privilege, or downstream service exposure.

Security Findings and Attack Paths

• observed — The security candidate is deferred, not a verified finding. Effective repository merge settings and active rulesets remain unavailable, so neither an enforcement mismatch nor its introduction by this PR is established.

Trust Boundaries and Controls

• observed — The documented signing paths associate human commits with a registered SSH signing key and verified account email, while automation uses API-created commits signed by GitHub. Whether deployed repository controls enforce these expectations remains unresolved.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the documentation change to add a signed-commits section to CONTRIBUTING.
Description check	✅ Passed	The description is directly related to the documentation change and explains the signing policy and contributor guidance.
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
Then checks the branch before the merge
SSH keys and verified mail
Keep unsigned changes off the trail
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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 141-142: Update the unsigned-commit guidance to say to re-sign the
commits and force-push the rewritten history to the existing PR branch; remove
the instruction to recreate the branch and open a new PR.

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🔇 Additional comments (1)

.github/CONTRIBUTING.md (1)

143-143: 🔒 Security & Privacy | 🛡️ Detected with Advanced Tier

Establish the effective rebase setting before relying on this statement.

If allow_rebase_merge is enabled, a repository writer can select rebase-and-merge for a pull request containing unsigned commits. GitHub can then create commits without signature verification. Confirm that the repository settings and active rulesets disable rebase-and-merge. If they do not, disable it or revise this statement.