Adds (never weakens existing) cases, each in its own temp AGENT_QUEUE_ROOT using
the no-op engine stub:
- backward-compat: legacy engine/cwd/yolo-only .md still lands in review/.
- priority: with --max 1, a critical job queued after a low job runs first
(order-recording stub).
- capability mismatch: has:definitely-not-installed -> failed/
result=capability_mismatch, asserting the agent was never launched.
- engine-class: agentic-coder + no engine, DEVIN_BIN stubbed -> review/.
- idempotency: same key+body twice -> 1 inbox file; same key+changed body in
inbox -> superseded; same key+different body after drain -> rejected.
Inbox counts use find (not a globbing ls) so set -e/pipefail tolerate an empty inbox.
