Merge queue: tested states are deleted once their entry leaves
Each entry's combined state is pushed to g1t-queue/<entry> for its checks and merge_group workflows, and nothing ever removed it, so a busy repository's branch list filled with them (hello had 19). The queue now deletes the branch when an entry lands, fails or is taken out, as GitHub does with gh-readonly-queue/*. The repos service gains delete_branch, which speaks receive-pack with delete-refs and refuses any name not starting with g1t-, so it can never remove a person's branch. A failed delete only logs.
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