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/paths.rs

@@ -219,11 +219,12 @@ pub fn shared_root() -> PathBuf {
 pub const KNOWLEDGE_DIR: &str = "/var/lib/hyperhive/knowledge";
 
 /// `gateway/` — generated nginx include fragments for the gateway vhost.
-/// The gateway container bind-mounts *this subdir only* (not the whole
-/// state root) at `/run/hive-state/`, so nginx can read `agents.conf`
-/// without the rest of `/var/lib/hyperhive/` (forge/matrix tokens, etc.)
-/// being exposed to the gateway container.
-// nix: bind-mounted into the gateway container (hive-gateway.nix) — must match.
+/// nginx runs on the host and reads this path directly; it used to be
+/// bind-mounted into a gateway container at `/run/hive-state/`, exposing
+/// this subdir *only* so the rest of `/var/lib/hyperhive/` (forge/matrix
+/// tokens, etc.) stayed out of reach. On the host that narrowing is the
+/// unit's sandbox, not a mount — nginx is not confined by this path.
+// nix: named by the gateway's nginx config (hive-gateway/vhosts.nix) — must match.
 #[must_use]
 pub fn gateway_dir() -> PathBuf {
     state_root().join("gateway")