Fix agent archive lifecycle reset

[cbusillo]

Summary

• preserve same-session archived terminal agents across a UI sender reconnect gap
• drop archived terminal agents from older sessions when a fresh sender set registers
• cover same-session reconnect, second live session registration, and stale archive pruning with focused tests

Validation

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