Inbox replies to managed agents can disappear before delivery

## Summary
A human reports that replying to a managed agent through **Inbox** can cause the new DM message to appear briefly in the conversation and then disappear before the agent handles it.

## Expected
An Inbox reply should persist in the DM timeline and be delivered once to the managed-agent harness (or show an explicit delivery/error state).

## Actual
The reply is shown in the agent DM, but sometimes the agent does not process it and the message later disappears from the timeline.

## Steps reported
1. Open an Inbox notification for a managed agent's message.
2. Reply from Inbox.
3. Observe the reply appear in the agent DM.
4. In the failing case, the agent never responds and the reply disappears.

## Evidence and scope
- Reported from the macOS Buzz app on 2026-08-11 around 23:30 Pacific.
- This was **not reproduced deterministically**.
- A control Inbox reply did arrive at the harness with the expected channel (`h`) and agent mention (`p`) tags, so the failure may be specific to an Inbox-reply reconciliation path or an intermittent delivery race rather than all DM delivery.
- No private message content or account identifiers are included here.

## Suggested investigation
Trace one Inbox reply event end-to-end: local optimistic insertion -> signed event submission -> relay acknowledgement/storage -> DM subscription/cache reconciliation -> harness receipt/enqueue. Preserve the event id at each stage, and ensure a failed delivery is surfaced rather than removing the message.