Toggle the loop's permission phase.
The loop treats ctx.phase as the canonical permission mode. This tool is
how the model requests a transition:
"enter"— switch to:plan(read-only). Loop typically enforces this by rejecting mutation tools."exit"— switch to:default. When the run was STARTED in:plan(host-pinned read-only), the permission layer requirescan_use_toolapproval before this tool ever executes, and denies the call outright when no callback is configured — the model cannot lift a host-imposed read-only restriction on its own. When the model itself entered plan mode from a looser phase, exiting merely restores the original grant and needs no approval. SeeExAthena.Permissions.
The tool doesn't mutate ctx directly — ctx is immutable per-call. It
returns a {:phase_transition, new_phase} sentinel that the loop consumes
and applies to subsequent iterations.