Moves the "Harness systemd unit shape" section from
docs/agent-lifecycle/agent-roster.md into docs/turn-loop/README.md: it
describes the per-agent harness systemd unit (env vars, PATH wiring,
serviceConfig), which is turn-loop material, not roster material.
Fixes two facts while moving: the ExecStart package is `hive-agent`,
not `hyperhive` (no package by that name exists); and `ruth.nix`
doesn't set any forge subscription default — it only defaults
`services.hyperhive.agent.docs.enable`.
Updates the inbound pointers in docs/turn-loop/config.md and the
module comment at nix/agent-modules/agent-service.nix.
Refs #3902
2b2608a491