fix(selector): queue sessions by agent attention

[abeljim]

Summary

• make agent-attention priority authoritative, including the current session
• order states as Blocked, Done-unseen, Stopped, Running, then None
• preserve FIFO order within each activity bucket as session state changes
• seed initial FIFO positions by session age instead of the host name-sorted snapshot

Stack

• based on fix(remote): stabilize upgrade health and invoke updater #24

Testing

• cargo test -p flock-selector (127 passed)
• cargo fmt --all -- --check
• git diff --check