Why: Q2 acceptance requires leader runs to detect stuck delegation loops and
cap replans instead of spinning to the turn limit, and a mailbox primitive
whose messages can never resolve a pending permission approval (acceptance 3).
Evidence: squad conformance tests (leader_mailbox_conformance_test.go):
ledger stuck->replan, replan cap finalize, mailbox delivery after hold,
mailbox-cannot-approve-permission (structural + actor second defense),
post guards; full suite green with -race.
Impact: LeaderState gains Progress/StuckTurns/Replans/MaxReplans/ReplanNote
(persisted in leader.json); RunState gains Mailbox (persisted in
runstate.json); RespondDecision gains Actor - non-human actors are rejected
on choice gates; step briefs and leader task briefs include pending mail.
Refs: Q2 plan WP2 (2026-10-07-agent-stack-q2q3-plans.md); task card
.task/feature-agent-stack-q2.md
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