Make signature CI pull-request aware on the event branch

[robobryce]

Summary

Port the signature-CI fix from #229 to event/2026-07-cscs-summer-school so event-based PRs such as #227 and #228 run the corrected workflow.

The old workflow compares every branch to main, causing event PRs to fail on inherited event history. This version selects the actual PR commits for fork PRs and pull-request/<number> mirrors, with safe fallbacks for non-PR pushes and merge groups.

This commit contains the same two workflow blobs as #229.

Validation

• workflow blobs match Make signature CI pull-request aware #229 byte-for-byte
• fork PR and mirror selection simulations
• truncated-payload and API-error fail-closed simulations
• cross-repository comment suppression and base-aware remediation simulation
• yamllint
• pre-commit and pre-push hooks
• git diff --check

[copy-pr-bot]

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[github-actions]

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