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.

docs/web-ui/dashboard.md

@@ -180,10 +180,12 @@ omitted — agents share the host netns, so there is no per-container net
 counter (per-agent network needs the netns-isolation roadmap in
 `docs/network.md`).
 
-**1NFR4** — start / stop / restart the four hive infrastructure
-containers (`hive-ci`, `hive-forge`, `hive-gateway`, `hive-matrix`)
-directly from the dashboard, without needing an `infra_admin` agent's
-`restart` MCP tool. One row per container: name, a `badge-ok`/`badge-fail`
+**1NFR4** — start / stop / restart the four hive infrastructure services
+(`hive-ci`, `hive-forge`, `hive-gateway`, `hive-matrix`) directly from the
+dashboard, without needing an `infra_admin` agent's `restart` MCP tool.
+Three are containers; `hive-gateway` is the host's `nginx.service`, and is
+the one an agent may **not** restart — this panel is the way it gets
+bounced. One row per service: name, a `badge-ok`/`badge-fail`
 running/stopped dot, and `↺ R3ST4RT` + `■ ST0P` (running) or `▶ ST4RT`
 (stopped) buttons, same themed-confirm pattern as the K3PT ST4T3
 tombstone actions. Backed by
@@ -460,7 +462,7 @@ The current capabilities are:
 | `manage_root_agent` | allows the `set_status` / lifecycle tools on the root agent |
 | `read_host_journal` | unlocks `get_host_journal` to read journald from inside a container |
 | `query_agent_state` | allows `get_loose_ends(agent: "<name>")` calls targeting other agents |
-| `infra_admin` | allows `restart` on hive infrastructure containers (`hive-ci`, `hive-gateway`, `hive-forge`); each restart is logged to the AUDIT trail |
+| `infra_admin` | allows `restart` on hive infrastructure containers (`hive-ci`, `hive-forge`, `hive-matrix`; the gateway is operator-only); each restart is logged to the AUDIT trail |
 
 Each row is one agent. Columns are the capability names returned by
 `GET /api/capabilities` as `caps: Vec<String>`. Checking or unchecking
@@ -1340,9 +1342,11 @@ below — some endpoints aren't in it yet.
   rootfs every ~5 min, `-x` excluding the shared read-only nix store.
   `null` until the first sample lands.
 - `POST /api/infra-container/{name}/{action}` — start / stop / restart a
-  hive infra container (C0R3 › 1NFR4 panel). `name` parses into the
+  hive infra service (C0R3 › 1NFR4 panel). `name` parses into the
   `InfraContainer` allowlist (`hive-ci`/`hive-forge`/`hive-gateway`/
-  `hive-matrix`, 400 on unknown), `action` ∈ `start|stop|restart`. Calls
+  `hive-matrix`, 400 on unknown), and the variant decides the unit —
+  `container@<name>.service`, or `nginx.service` for the gateway.
+  `action` ∈ `start|stop|restart`. Calls
   the same `priv_client::control_infra_container` helper the
   `infra_admin` agent path uses; records an `audit_log` entry
   (`start_infra`/`stop_infra`/`restart_infra`, actor `"operator"`) either