Preserve legacy agent visibility while pruning stale archives

[cbusillo]

Summary

• treat ownerless legacy agents as visible to session-scoped model-facing queries
• prune archived terminal agents from disconnected sessions whenever sender registration changes, not only after a full sender gap
• keep same-session reconnect archives and connected-session archives intact

Validation

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