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-agent-mcp/src/mcp/args.rs

@@ -326,7 +326,8 @@ pub struct GetHostJournalArgs {
     /// nspawn machine name verbatim (e.g. `h-iris`). Omit for host journal.
     /// Agent containers use the `h-` prefix (e.g. `h-iris`); infrastructure
     /// containers use their full name (e.g. `hive-ci`, `hive-forge`,
-    /// `hive-matrix`, `hive-gateway`).
+    /// `hive-matrix`). The gateway has no machine — its nginx runs on the
+    /// host, so read it with `unit: nginx.service` and no `container`.
     #[serde(default)]
     pub container: Option<String>,
     /// Number of lines to return (default 30, max 100).

hive-agent-mcp/src/mcp/mod.rs

@@ -681,8 +681,9 @@ impl AgentServer {
             Only succeeds if `name` is a direct child of this agent in the topology \
             tree — the server enforces this. No approval required. \
             Agents holding the `infra_admin` capability may also pass a hive \
-            infrastructure container name (`hive-ci`, `hive-gateway`, `hive-forge`) \
-            to restart it directly via the privileged helper."
+            infrastructure container name (`hive-ci`, `hive-forge`, `hive-matrix`) \
+            to restart it directly via the privileged helper. The gateway is \
+            not restartable by an agent — ask the operator."
     )]
     async fn restart(&self, Parameters(args): Parameters<RestartArgs>) -> String {
         let log = format!("{args:?}");
@@ -790,7 +791,8 @@ impl AgentServer {
             `container`: nspawn machine name verbatim (e.g. `h-iris`). Omit for host journal. \
             Agent containers use the `h-` prefix (e.g. `h-iris`, `h-atlas`); \
             infrastructure containers use their full name (e.g. `hive-ci`, `hive-forge`, \
-            `hive-matrix`, `hive-gateway`). \
+            `hive-matrix`). The gateway is not a container: its nginx logs are in \
+            the host journal, so omit `container` and pass `unit: nginx.service`. \
             `lines`: how many lines (default 30, max 100). \
             `priority`: minimum syslog level enum. \
             `grep`: regex matched against log message fields (journalctl --grep). \