merge-side-checkout
A deny rule: it refuses a checkout of one merge side over a conflicted file, which discards the merge, and names what to run instead.
What it catches
A checkout of one merge side over a conflicted file, which discards the merge.
Why
It reads like "undo my edit to this file" and is not: during a merge the working-tree copy IS the merge, and this replaces it wholesale with one side. Measured here: git checkout MERGE_HEAD -- magusfile.buzz during a conflict resolution silently dropped the branch's own half of a merged feature. Nothing failed, the gate stayed green, and the feature could not fire until someone read the code days later. For a generated file, magus vcs resolve settles every conflicted one by regenerating. To take one side deliberately, say which: git checkout --ours or --theirs. To keep the merged result, it is already in the file.
Seeing it
A verdict names its rule in brackets, which is how you got here:
deny [merge-side-checkout]: ...
magus describe rule merge-side-checkout prints the same entry at a terminal, and
magus describe rules lists every rule this workspace enforces.