From 005b-golden.md's task table (design 005b).
- Covers: #850 item 1
Goal: Box.can_merge requires Godot on a premise #847 falsified (an
ordinary merge no longer runs probe_gate.py). Implement from **#850 item
1**: either relax the predicate or keep it and restate the reason as "you
could not sweep what you merged" — update gate_note and
test_spawn_dev.py:1034 to match. Re-read the predicate after T2 lands,
since T2 changes the premise again (a merge *may* run probes).
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