feat(3088): move the gateway's nginx + dnsmasq onto the host

The gateway's nginx + dnsmasq no longer run in their own nspawn container.
`nix/host-modules/hive-gateway/default.nix` loses the
`containers.hive-gateway` wrapper and everything that existed only to punch
holes in it: `privateNetwork = false`, `CAP_NET_ADMIN`, five bind mounts,
its own `stateVersion`, `networking.firewall.enable = false`,
`networking.resolvconf.enable = false`, and the `hive-gateway-resolv`
path+service pair. 465 -> 303 lines.

The container never bought isolation here. It shared the host netns by
necessity — nginx binds the host's :80/:443, dnsmasq answers on the bridge —
so each of those settings was undoing a boundary the gateway could not
afford in the first place.

Four things made it more than a deletion, none of them visible in the nix
diff:

- The self-signed cert service also imports the hive CA leaf, so removing it
  with the container would have left nginx naming a missing cert file, which
  it refuses to load at all.
- The nginx reload is a hive-priv verb. It still needs root, but no longer
  for the reason its doc gave, and `--machine=` was both transport and
  scope — so the unit name is now hard-coded in the helper as the
  containment.
- The lifecycle verb named a container that stops existing.
- `journalctl -M hive-gateway` had no machine to enter.

Per the operator's ruling, the operator verb keeps working and agents lose
it. `InfraContainer` answered three questions that used to share an answer;
it now splits into `name()` (identity), `target()` (Container vs HostUnit),
`service_unit()` (the systemd unit), and `agent_restartable()`, which the
MCP restart path checks before the capability so the refusal cannot read as
"ask for infra_admin". `SIBLING_CONTAINERS` drops the gateway — it gates the
requests that name a container as a string — while `FromStr` still accepts
it, because that answers what a name is, not who may act on it. The
dashboard's gateway journal reads host journald filtered to `nginx.service`.

Prose was corrected where it only named a location, and re-argued where the
container was doing security work: a `0666` per-agent socket was safe
because only the gateway container had the directory bind-mounted. There is
no mount now, so the directory permissions are the whole of the access
control — the constraint holds, its mechanism doesn't.

Gate: nix fmt / clippy --all-targets -D warnings / cargo test all clean (710
tests); hivectl-cli.md regenerated from the clap tree. The nix eval was run
in both TLS shapes at this commit: every delta in the rendered
virtualHosts is one of the three intended path moves, dnsmasq settings are
byte-identical, and the absence probe flips true -> false with bindMounts
emptied.

hive-c0re/src/priv_client.rs

@@ -477,20 +477,21 @@ pub async fn register_ci_runner(token: &str) -> Result<()> {
     .await?)
 }
 
-/// Restart a hive infrastructure container on the host (thin wrapper over
-/// [`control_infra_container`] with `action = Restart`). hive-priv
-/// re-validates `container` against its root-side allowlist; callers must
-/// already have checked the requesting agent holds the `infra_admin`
-/// capability.
+/// Restart a hive infrastructure service on the host (thin wrapper over
+/// [`control_infra_container`] with `action = Restart`). Callers must
+/// already have checked that the requesting agent holds the `infra_admin`
+/// capability *and* that the target is
+/// [`agent_restartable`](InfraContainer::agent_restartable).
 pub async fn restart_infra_container(container: InfraContainer) -> Result<()> {
     control_infra_container(container, InfraAction::Restart).await
 }
 
-/// Start / stop / restart a hive infrastructure container (`hive-ci`,
+/// Start / stop / restart a hive infrastructure service (`hive-ci`,
 /// `hive-gateway`, `hive-forge`, `hive-matrix`) on the host via `systemctl
-/// <action> container@<container>.service`. The [`InfraContainer`] enum is
-/// the allowlist — hive-priv needs no name re-validation. Used by the
-/// hive-wide `hivectl stop` / `hivectl start` flow.
+/// <action> <unit>`, where the unit is derived root-side from the variant
+/// (`container@<name>.service`, or `nginx.service` for the gateway). The
+/// [`InfraContainer`] enum is the allowlist — hive-priv needs no name
+/// re-validation. Used by the hive-wide `hivectl stop` / `start` flow.
 pub async fn control_infra_container(container: InfraContainer, action: InfraAction) -> Result<()> {
     ok(call(&PrivRequest::ControlInfraContainer { container, action }).await?)
 }