fix(ci): skip maintainer approval in merge queue

[purp]

Summary

Skip maintainer approval evaluation for merge-group checks while preserving the existing check name and trigger so GitHub can satisfy the queue's required check. Use the event context to run the approval job only for open PRs; review changes after queue admission do not cause a merge-group approval reevaluation.

Related Issue

No issue, as requested by the maintainer.

Changes

• Skip the job for merge-group events and closed PRs using the built-in GitHub context.
• Remove the obsolete merge-queue ref parsing. The existing review lookup and approval helper remain unchanged.

Testing

• git diff --check passes.
• Extracted workflow shell syntax passes bash -n.
• Actionlint and the pytest suite were unavailable because dependency downloads were blocked in the environment. No persistent tests added; no runtime changes requiring E2E tests.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs: not applicable.

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