Rationale
Parallelism without isolation is how two patches for the same change came to exist dozens of times over. One task per checkout and visible overlap are the cheapest isolation there is.
Required behaviour
Keep one active task per checkout, and treat a scope claimed by another worktree as a boundary to report, not to cross.
Failure behaviour
Nothing decides this rule directly, so a violation is reported by nobody; it is the rules that depend on it that decide what a machine can decide and stop or warn accordingly.
Verification
The enforced rules whose depends_on names this one carry the validators, the tests and the CI wiring. majordomus doctrine list shows them.