arm_pr_automerge.sh: GUARD_CHECK_NAME=guard-tree is stale; no PR head has that check run, so it refuses every PR #5846

arm_pr_automerge.sh defaults GUARD_CHECK_NAME to guard-tree and reads commits//check-runs. guard-tree is now a section inside x86-main, not a check run: gh api repos/paiml/aprender/commits/7f2eba6e80/check-runs has no guard* name, so every PR is refused 'absent' (rc 3). Found via mq_feeder (#5843), which works around it with GUARD_CHECK_NAME=x86-main.

Fix: change the default to the check run that carries guard-tree (x86-main), update the --self-test fixtures, and drop the workaround in mq_feeder.sh. Gate content: needs 2 non-author seats at the final head.

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