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
  • Added guidance requiring commits to the default branch to be signed, with unsigned pushes refused.
  • Explained signing options for people, interactive agents, apps, bots and workflows.
  • Clarified that unsigned commits on pull request branches block merging, squash merges are required, and rebase merges are unavailable.

Walkthrough

The contribution guide now describes signed-commit requirements, commit methods for people and automated systems, and pull request merge requirements.

Changes

Signed commit guidance

Layer / File(s)	Summary
Commit and merge requirements .github/CONTRIBUTING.md	The guide requires signed commits, describes SSH signing and API-based commit creation, and specifies squash merges. It states that unsigned commits on a pull request branch block merging and that rebase-merge is disabled.

Priority: ⬇️ Low

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

Change: Other

Merge Risk: 🔵 Low · up to 4ae7e

Contributors may create an unnecessary new PR when correcting unsigned commits. Update the guidance to preserve the existing PR where force-pushing is allowed; this is a bounded documentation issue.

Architecture Summary

Architecture risk: 🔵 Low · up to 4ae7e

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: Added guidance requiring signed commits, specifying SSH signing and verified-email requirements for people and interactive agents, API-based commit creation for apps, bots and workflows, and squash merges. It also states that unsigned PR-branch commits block merging and rebase-merge is disabled.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the documentation change: adding a Signed commits section to CONTRIBUTING.
Description check	✅ Passed	The description is directly related to the changeset and explains the signing guidance, contributor scope, merge requirements, policy basis, and docs-only nature of the change.
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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Actionable comments posted: 1

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Inline comments:
Review comments at @.github/CONTRIBUTING.md:
- Line 87: Update the contributor guidance around `git cherry-pick -S` to direct
contributors to force-push rewritten, signed commits to the existing PR branch
when allowed; recommend opening a new PR only when a separate rule blocks that
force-push.

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