throwaway-copy
A deny rule: it refuses a run inside a temp or scratchpad copy, which leaves the real tree unverified, and names what to run instead.
What it catches
A run inside a temp or scratchpad copy, which leaves the real tree unverified.
Why
A run inside a temp or scratchpad copy judges a tree nobody ships: a green gate leaves the real tree unverified, generated files land in the copy, and the cache splits. No magus run needs a clean tree; run from the workspace and name the project. If you genuinely need a pristine tree, use a throwaway git worktree add, not a copy.
Seeing it
A verdict names its rule in brackets, which is how you got here:
deny [throwaway-copy]: ...
magus describe rule throwaway-copy prints the same entry at a terminal, and
magus describe rules lists every rule this workspace enforces.