poll_messages now collects all queued messages, returns them,
then deletes them from sled. No more duplicate delivery.
This is correct for store-and-forward: once the client receives
the messages, the server's job is done. If the client crashes
before processing, the messages are lost — acceptable for Phase 1.
Phase 2 can add explicit ack-based delivery if needed.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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