Adds hyperhive_rev to the dashboard's /api/state StateSnapshot,
resolved via the same current_flake_rev helper get_agent_meta's
per-agent hyperhive_rev already uses. home.js renders it next to the
existing hive-identity line, truncated to the last 12 chars with the
full value in title=, hidden when the flake ref isn't a local path pin.
Requested by annika (infra.run) via dmatrix, hyperhive#2854.
761f4b8351