Fix background result relay delivery

[jamiepine]

Stacked on #651.

Summary

• confirm replies, questions, and files at the messaging adapter before acknowledging background results
• retry failed result relays once, keep exhausted results queued, and retry them after the next user turn
• discard retrigger scaffolding and failed reply text instead of persisting internal instructions in conversation history
• terminate completed file deliveries cleanly and reject unsupported adapter payloads

Delivery transitions converge as pending -> in flight -> delivered or pending -> in flight -> retry pending -> deferred. Adapter calls are capped at 30 seconds and receipt waits at 60 seconds. Duplicate completion batches remain ordered in the deferred queue, and only confirmed deliveries mark status items relayed.

Testing

• just preflight
• just gate-pr
• cargo test agent::channel_history::tests
• cargo test tools::send_file::tests
• cargo test routed_sender_
• adapter tests for Mattermost, Portal, and Twitch

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