Why QuitSelf doesn’t proceed at once (§6.3). Until Phase 2b’s override
window exists, every one of these is a refusal (spec §9: the non-user path
ships with 2b).
§4.2: the tool call is part of the turn that asked for it, so quit when
that turn ends (or after SELF_QUIT_TURN_GRACE), not now. Returns false
when a quit is already scheduled or under way.