Dead letter notifications via Queue to sender's proxy

[vrknetha]

Problem

When a message hits the dead letter queue on the recipient's side, the sender currently has no reliable way to know. The receipt POST (status: dead_lettered) is fire-and-forget — if it fails, the sender never learns their message was permanently undeliverable.

Proposed Flow

Alice's inbox → message dead-lettered
  → Alice's connector POSTs receipt (status: dead_lettered) to Alice's proxy
  → Alice's proxy publishes to Queue
  → Bob's proxy consumes → stores in Bob's DO
  → Bob's DO pushes to Bob's connector via WebSocket
  → Bob's OpenClaw agent receives dead letter notification
  → Agent can: retry, alert human, try alternative route

Why This Matters

Silent message loss is the worst failure mode for agent-to-agent communication. The sender's agent needs to know so it can:

• Retry the message
• Alert the human operator
• Try an alternative communication route
• Log the failure for debugging

Architecture

• Same Queue infrastructure as Route delivery receipts through proxy Queue instead of direct HTTP #168 (delivery receipts)
• Dead letter receipts are just receipts with status: dead_lettered + reason
• No separate Queue needed — same receipt Queue, different status field
• Connector interface unchanged — POST to own proxy

Dependencies

• Delivery receipts are stored but never consumed by sender #165 (WebSocket receipt push to connector)
• Route delivery receipts through proxy Queue instead of direct HTTP #168 (Queue-based receipt routing)

Impact

• Minimal additional Queue operations (dead letters are rare)
• Transforms silent failures into actionable events