docs: add Signed commits section to CONTRIBUTING (#125)

Adds a **Signed commits** section to this repo's CONTRIBUTING, per owner
ruling **D218**. The estate policy is
[`docs/SIGNING-POLICY.adoc`](https://github.com/hyperpolymath/standards/blob/main/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.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01WRvDivYwLSeVCJUrfjic3f

---------

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

.github/CONTRIBUTING.md

@@ -119,3 +119,20 @@ Footer: issue reference, e.g. Closes #123
 \[optional body\]
 
 \[optional footer\]
+
+### Signed commits
+
+Every commit that reaches the default branch must be signed; a ruleset refuses
+unsigned pushes. Estate policy:
+[SIGNING-POLICY](https://github.com/hyperpolymath/standards/blob/main/docs/SIGNING-POLICY.adoc).
+
+- **People and interactive agents** sign with an SSH key registered on GitHub
+  as a *signing* key (`gpg.format=ssh`, `user.signingkey=<key>.pub`,
+  `commit.gpgsign=true`). The committer email must be verified on that account.
+- **Apps, bots and workflows** never `git push` local commits. They write
+  through the API (`createCommitOnBranch` or the estate `signed-push` action)
+  so that GitHub signs each commit.
+- Merge PRs with **squash**. GitHub signs the resulting squash commit, and the
+  PR's individual commits do not reach the protected default branch.
+  Contributors should still sign their own commits as required by estate policy.
+  Rebase-merge replays commits unsigned and is disabled.