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/lifecycle/mod.rs

@@ -604,14 +604,14 @@ pub async fn is_running(name: &str) -> bool {
         .is_ok_and(|s| s.success())
 }
 
-/// True when a hive infrastructure container's systemd unit is active.
-/// Sibling of [`is_running`] for sub-agents, but infra container/unit names
-/// (`hive-ci`, …) already have no `h-` prefix to strip, so this queries
-/// `container@<unit_name>.service` directly rather than going through
-/// [`container_name`]. Used by the dashboard C0R3 page's 1NFR4 sub-tab to
-/// show each infra container's live status dot.
+/// True when a hive infrastructure service's systemd unit is active.
+/// Sibling of [`is_running`] for sub-agents, but infra names (`hive-ci`, …)
+/// have no `h-` prefix to strip and are not all containers, so the unit
+/// comes from the variant itself rather than from [`container_name`]. Used
+/// by the dashboard C0R3 page's 1NFR4 sub-tab to show each one's live
+/// status dot.
 pub async fn infra_is_running(container: hive_priv_sock::InfraContainer) -> bool {
-    let unit = format!("container@{}.service", container.unit_name());
+    let unit = container.service_unit();
     Command::new("systemctl")
         .args(["is-active", "--quiet", &unit])
         .status()