mail check: say WHY a message is re-delivered (lease expiry of unacked cur/ mail) + make the ack protocol discoverable #340

What happened

An agent that reads its inbox with tps mail check <agent> and never acks sees its entire cur/ re-delivered on every check ≥30 min after the previous one. checkMessages() (packages/cli/src/utils/mail.ts ~L87–97) walks cur/ after draining new/ and re-returns any message that is not read/nackedAt, not waiting on retryAfter, and whose checkout lease has expired (LEASE_TIMEOUT_MS = 30 min), re-stamping checkedOutAt.

That is correct at-least-once semantics — but nothing in the output says why a message is back, so it reads as a replay/duplication bug. It cost two hours chasing a phantom "mover" (relay, archiver, watchers were all suspected and excluded one by one) before the code path was found. The 2026-08-29 "60 accumulated in cur/, replayed as 66 unread" was the same thing.

Hook/plugin-driven agents never hit this because the hook acks. Any agent (or human) reading via mail check directly does.

Asks

1. Say why. When checkMessages re-delivers from cur/, mark it in the output — e.g. 📬 kern → flint … (re-delivered: lease expired, attempt 3, unacked since 2026-09-13T22:27Z) — and log a distinct event (redeliver) rather than nothing.
2. Make the protocol discoverable. tps mail check for an agent with no hook/plugin binding should print a one-line notice the first time it re-delivers: "N messages re-delivered because they were never acked; run tps mail ack <id> [agent] or tps mail check --ack."
3. Optionally: a --ack flag on mail check (ack on read) for non-hook consumers.

Evidence

• LEASE_TIMEOUT_MS = 30 * 60 * 1000; isLeaseExpired; ackMessage sets read+ackedAt and removes the file from cur/.
• Observed as strict 0/102 alternation on a 15-minute check cadence, zero re-deliveries on a 2-minute cadence, 106 after a 3-hour gap; a 15s trap on new/ never fired because the files never leave cur/.
• Fixed on the consumer side by acking (99 acked → cur/ 0 → check returns 0). Refs dtrt-dev/ops ops-8mhg.