docs: correct Signed commits section (squash does not sign bot commits)

[hyperpolymath]

Corrects the Signed commits section added to this repo's CONTRIBUTING under owner ruling D218.

The first version said a squash merge is one way for apps and bots to get signed commits. That is wrong. GitHub signs the squash commit, but required_signatures checks every commit on the PR branch before it allows the merge. A single unsigned commit on the head blocks the PR; this was measured on pons-asinorum #46, where gh pr merge --squash was refused with "base branch policy prohibits". CodeRabbit flagged the same point on rpa-elysium#142.

The corrected section:

• drops "or a squash merge" from the ways bots can get signed commits; the API (createCommitOnBranch) and the estate signed-push action remain;
• says the ruleset checks every commit on the PR branch, and explains how to recover (re-create the branch with git cherry-pick -S and open a new PR).

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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