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/src/mcp_config.rs

@@ -60,8 +60,8 @@ fn allowed_capability_tools() -> Vec<String> {
         match t.as_str() {
             "read_host_journal" => tools.push("get_host_journal".to_owned()),
             // infra_admin lets an agent restart hive infrastructure
-            // containers (hive-ci / hive-gateway / hive-forge) through the
-            // existing `restart` tool. Unlock it here so agents that hold
+            // containers (hive-ci / hive-forge / hive-matrix — not the
+            // gateway) through the existing `restart` tool. Unlock it here so agents that hold
             // the capability without the full `lifecycle` group can still
             // call it; c0re re-checks the capability server-side and only
             // honours infra-container names via this path.

hive-agent/src/web_ui/mod.rs

@@ -169,11 +169,12 @@ pub async fn serve(
 /// Best-effort unlinks any stale socket left from a crashed previous
 /// harness (clean exit removes it, but `bind(2)` refuses to overwrite
 /// an existing file) and `mkdir -p`s the parent for first-boot. Mode
-/// `0o666` — world-accessible so the gateway container's nginx process
-/// can `connect(2)` without sharing a group with the agent user.
-/// The per-agent subdir (`/run/hive-agent/<name>/`) is only accessible
-/// to containers that have it bind-mounted, so world-accessible sockets
-/// are not a material risk.
+/// `0o666` — world-accessible so the gateway's nginx process can
+/// `connect(2)` without sharing a group with the agent user. What bounds
+/// that is the per-agent subdir (`/run/hive-agent/<name>/`): the socket
+/// mode grants everyone, the directory decides who gets to ask. It used
+/// to be bind-mounted into the one container that needed it; nginx is a
+/// host unit now, so the directory's own permissions are the whole story.
 ///
 /// Marker-gating + the gateway-side consumer: see
 /// [`docs/gateway.md::Per-agent unix-socket upstream`](../../../docs/gateway.md).