fix(ci): exclude the promote job's own check runs from promotion validation

[laurenhitchon]

First live promotion attempt surfaced a self-referential bug: when the target is the current main head, the promote job's own check run attaches to that commit — so validation counted itself as pending (guaranteed failure), and the failed attempt then sat on the commit as a red check, poisoning every retry. The green-check gate now filters out check runs named promote (comment notes the rename coupling). Everything else about the gate is unchanged and worked as designed.

[DFSIdigital]

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[Copilot]

Pull request overview

This PR fixes a self-referential gating issue in the promote-v1 workflow where the promotion validation could incorrectly count the promote job’s own check run(s) on the target SHA, causing the gate to see itself as pending (and potentially blocking retries due to accumulated failures on that check name).

Changes:

• Update the check-runs query to filter out check runs named promote before computing success/failed/pending counts.
• Add inline documentation explaining the self-reference failure mode and the coupling to the check-run name.

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