The host admin socket `/run/hyperhive/host.sock` was `0660 root:root` (no
SocketGroup), so hivectl needed sudo. Group-own it by a new `hive-admin`
group and add a `services.hyperhive.c0re.adminUsers` allowlist: listed users
join `hive-admin` and drive hivectl without root.
- `SocketGroup = "hive-admin"`, `SocketMode = "0660"` on the hive-c0re.socket
unit.
- `/run/hyperhive` -> `0751` (traverse-only, no listing) so the group can reach
the socket path; the socket's own `0660 hive-admin` mode gates the
connection, and the per-agent subdirs keep their own restrictive perms.
- Empty `adminUsers` (the default) leaves `hive-admin` memberless -> root-only,
as before.
The admin socket is full hive control (spawn/kill/destroy/deploy), so
`adminUsers` is an explicit, opt-in trust grant. Documented in
docs/boundary.md (host admin socket access) + docs/tools/hivectl.md.
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