Bound active work and support GitHub merge queues

[philcunliffe]

Summary:

• cap non-frozen Neutral work surfaces at a configurable default of four
• keep healing existing work while pausing new branch-producing intake at capacity
• freeze neutral:stuck work outside the cap and prevent autophagy from bypassing it
• add merge-queue mode that delegates BEHIND freshness to merge-group CI
• enqueue only reviewed-clean current heads and observe actual GraphQL queue membership
• make neutral-reconcile consume the deterministic admission and queue decisions

Verification:

• npm test (223 passing)
• npm run typecheck
• git diff --check

Design: LLP 0060