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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ℹ️ Recent review info

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Reviewing files that changed from the base of the PR and between e3edf30 and 8e1ab13.

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• .github/CONTRIBUTING.md

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🔇 Additional comments (1)

.github/CONTRIBUTING.md (1)

123-138: LGTM!

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📝 Summary

Summary by CodeRabbit

• Documentation
  • Added guidance requiring signed commits for changes reaching the default branch, including requirements for people, automation and pull requests.
  • Documented squash merging and how to handle unsigned commits.

Walkthrough

The contribution guide now requires signed commits on the default branch. It describes signing methods for people, interactive agents, apps, bots and workflows, and explains the commit checks and merge requirements for pull requests.

Changes

Commit signing requirements

Layer / File(s)	Summary
Signing and merge requirements .github/CONTRIBUTING.md	The guide describes signing commits with a registered SSH signing key and verified committer email, or using the GitHub commit API or signed-push action for apps, bots and workflows. It states that unsigned pull request commits block merging, requires squash merges, and describes recreating unsigned commits as signed commits in a new pull request.

Priority: ⬇️ Low

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

Change: Other

Merge Risk: ⚪ Minimal · up to 8e1ab

This documentation change appears ready to merge: checked-in guidance and automation align with it, and no concrete user-impacting defect was found. The external ruleset and estate policy were not independently verified.

Architecture Summary

Architecture risk: 🔵 Low · up to 8e1ab

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 signing requirements and instructions covering SSH key setup for people and interactive agents, API-based commits for apps, bots and workflows, and squash merging. The text states that unsigned PR-branch commits block merging, recommends recreating them as signed commits in a new PR, and notes that rebase-merge is disabled.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: adding a Signed commits section to CONTRIBUTING.
Description check	✅ Passed	The description directly explains the documentation change, its signing policy, and its repository context.
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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Then checks that every mark is there.
The squash brings changes to the branch,
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