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 (1)

23-38: LGTM!

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📝 Summary

Summary by CodeRabbit

• Documentation
  • Contribution guidance now states that commits reaching the default branch must be signed, using a registered SSH signing key and verified committer email for people and interactive agents.
  • Automated apps, bots, and workflows are directed to use the API or the designated signed-push action instead of pushing local commits.
  • Squash merges are required; unsigned commits on a pull request branch block merging, and rebase merges are unavailable.

Walkthrough

The contribution guide now documents signed-commit requirements for contributors and automated processes. It also requires squash merges and explains that unsigned commits block merging.

Changes

Contribution policy

Layer / File(s)	Summary
Commit signing and merge requirements .github/CONTRIBUTING.md	The guide specifies signing rules for people, interactive agents, apps, bots, and workflows. It states that unsigned commits block merging, requires squash merges, and notes that rebase-merge is disabled.

Priority: ⬇️ Low

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

Change: Other

Merge Risk: ⚪ Minimal · up to 6e941

No concrete merge-blocking issue was established, and the repository’s checked-in contract supports signed commits.

Architecture Summary

Architecture risk: 🔵 Low · up to 6e941

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 instructions for people, interactive agents, apps, bots and workflows. Requires squash merges, explains that unsigned commits on the PR branch block merging, and states that rebase-merge is disabled.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change: adding a Signed commits section to CONTRIBUTING.
Description check	✅ Passed	The description is directly related to the documentation changes. It explains the signing policy, affected contributor types, merge requirements, and scope.
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.
Squash the commits, keep signatures there,
No unsigned push can cross the verge.
The guide now sets the rules to share.}

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

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Commit to this branch
• Create a new PR