Preserve agent archives across UI reconnects

[cbusillo]

Summary

• distinguish the first event-sender lifecycle from a later reconnect after all senders disconnect
• preserve archived terminal agents across sender gaps so completed agent metadata remains queryable after reconnect/resume
• add a regression for reconnecting after the sender gap

Validation

• cargo test -p code-core reconnect_after_sender_gap_keeps_existing_archived_agents
• cargo test -p code-core registering_second_session_keeps_existing_archived_agents
• ./build-fast.sh

Follow-up to #199 auto-review finding for #175.