A role file is read into every subagent dispatch that names it, and
nothing measured its size. A new watch on the same 15-minute tick as
claude_md_watch records `hyperhive.agent.subagent_role.lines`, one
series per `<role>.md` under the state dir's `subagent_roles/`,
labelled `role` with the file stem. Metric only: no todo, no threshold.
Fixes#4558
