The working folder a run is admitted to (P1.5) and started in.
Identity is a [ProjectId] rather than a path: the project already carries a canonical root the host validates at launch, and an id is what a second device can name without being able to resolve it (D15, D16). The folder inside it is text a person chose, and is redacted like every path.
`None` is unreachable at admission. It exists so a request can be built before a scope is chosen and so the modal has a state to refuse; P1.5 makes it impossible to start a run with it. M4 adds git shapes - a worktree per run - as new variants, which is why this is an enum.
