process-poll
A deny rule: it refuses a process table inspected to wait on magus work the lock already reports, and names what to run instead.
What it catches
A process table inspected to wait on magus work the lock already reports.
Why
A magus run holds a project lock and announces itself, and magus status --watch=15s reads that same lock state continuously: holder PID, command, age. pgrep, pidof and ps invent a poll with no bound of its own that answers a question the lock message already answered.
Seeing it
A verdict names its rule in brackets, which is how you got here:
deny [process-poll]: ...
magus describe rule process-poll prints the same entry at a terminal, and
magus describe rules lists every rule this workspace enforces.