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 contribution guidance requiring signed commits for changes reaching the default branch, including instructions for signing, merging and handling unsigned commits.

Walkthrough

The contribution guide now requires signed commits to reach the default branch. It specifies signing methods for different contributor types and gives instructions for squash merges and branches that contain unsigned commits.

Changes

Signed commit guidance

Layer / File(s)	Summary
Signing and merge instructions .github/CONTRIBUTING.md	The guide specifies signing requirements for people, interactive agents, apps, bots and workflows. It instructs contributors to squash-merge and recreate branches with unsigned commits using signed cherry-picks. It also states that rebase-merge is disabled.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

Merge Risk: 🔵 Low · up to c0e9b

The guide may lead contributors to perform unnecessary cherry-picks and open replacement PRs. Correct this instruction before relying on it.

Architecture Summary

Architecture risk: 🔵 Low · up to c0e9b

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 guidance requiring signed commits to reach the default branch, specifying SSH signing requirements for people and interactive agents and API or signed-push commits for apps, bots and workflows. It also requires squash merges, directs contributors to recreate branches with signed cherry-picks when any PR-branch commit is unsigned, 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 change and explains the signing policy 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 that every key is there.
A squash merge brings the branch in line,
Signed cherry-picks keep commits fine.
No unsigned push shall cross the gate,
The guide makes signing clear and straight.}

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

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Actionable comments posted: 1

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Inline comments:
Review comments at @.github/CONTRIBUTING.md:
- Around line 119-138: Update the “Merge PRs with squash” guidance in the Signed
commits section to clarify that GitHub signs the squash commit reaching the
default branch, so unsigned intermediate commits on a PR branch do not require
branch recreation. Keep the note that rebase-merge is disabled.

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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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