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. The loop may require user approval (viacan_use_tool) before honouring this.
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.