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.

nix/host-modules/hive-gateway/options.nix

@@ -134,9 +134,9 @@ in
           and uses this cert, overriding the self-signed default — the
           auto-generated hive-CA-signed leaf is skipped entirely.
 
-          The directory is bind-mounted read-only into the gateway
-          container at `/run/hive-tls/`. nginx reads
-          `<certDir>/<tls.certName>` and `<certDir>/<tls.keyName>`.
+          nginx reads `<certDir>/<tls.certName>` and
+          `<certDir>/<tls.keyName>` directly — it runs on the host, so
+          the directory needs no bind mount and no copy.
           Default filenames (`cert.pem` / `key.pem`) match the output
           layout of nixpkgs's `security.acme` module.
 
@@ -232,9 +232,7 @@ in
         enabled, every request to the gateway's main vhost requires a
         valid username and password. nginx's built-in `auth_basic`
         module validates credentials against
-        `/var/lib/hyperhive/gateway/gateway.htpasswd` on the host
-        (exposed as `/run/hive-state/gateway.htpasswd` inside the
-        container via the existing gateway state bind-mount). Off by default.
+        `/var/lib/hyperhive/gateway/gateway.htpasswd`. Off by default.
 
         Manage users with `hivectl gateway create-user`, `delete-user`,
         and `list-users` — see `hivectl gateway --help` for usage.