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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CONTRIBUTING.adoc (1)

110-125: LGTM!

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📝 Summary

Summary by CodeRabbit

• Documentation
  • Updated the contribution guide with commit-signing requirements, guidance for people and automation, and steps for handling branches with unsigned commits.
  • Clarified that pull requests must be squash-merged; rebase-merge is disabled.

Walkthrough

The contribution guide now describes commit-signing requirements for commits reaching the default branch. It also sets out the permitted signing methods, squash-merge requirement, and steps for branches with unsigned commits.

Changes

Contribution guidance

Layer / File(s)	Summary
Signing and merge rules CONTRIBUTING.adoc	The guide requires signed commits and specifies signing methods for people, interactive agents, apps, bots and workflows. It requires squash merges, explains how to replace unsigned commits, and states that rebase-merge is disabled.

Priority: ⬇️ Low

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

Change: Other

Merge Risk: ⚪ Minimal · up to aa099

This PR adds commit-signing and merge guidance without changing enforcement. The default-branch signature rule is active; no specific mergeability failure is established, though the repository’s squash and rebase settings could not be confirmed.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the documentation change: adding a Signed commits section to CONTRIBUTING.
Description check	✅ Passed	The description directly explains the Signed commits documentation, its signing policy, and the docs-only 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
It picks signed commits for a fresh start
And leaves unsigned ones behind
The guide records the steps to take
Then hops along its way}

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