stage-all
A deny rule: it refuses a whole-tree git add (-A, -u, ., --all, --update), which sweeps in regenerated output, and names what to run instead.
What it catches
A whole-tree git add (-A, -u, ., --all, --update), which sweeps in regenerated output.
Why
A magus target writes its declared outputs as it runs, so the tree here is routinely dirty with files you did not edit. -A sweeps those and any build residue into a commit about something else, with no signal that it happened, and -u reaches the same outputs: it stages every TRACKED change across the whole tree, which is the same sweep minus files that are merely untracked, and a target's declared outputs are ordinarily tracked already. Measured: one such call put 69 files, a whole regenerated docs site plus five untouched source files, into a commit about four collection methods. magus vcs add classifies every dirty path against the declared output globs, keeps a source change and the outputs it produced together, and reports anything undeclared instead of staging it.
Seeing it
A verdict names its rule in brackets, which is how you got here:
deny [stage-all]: ...
magus describe rule stage-all prints the same entry at a terminal, and
magus describe rules lists every rule this workspace enforces.