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/boundary.md

@@ -121,8 +121,25 @@ The mode is load-bearing, not cosmetic. Write permission on a
 them, and the sticky bit is the only thing that would restrain that (it
 is not set here). A world-writable socket dir therefore lets anything
 able to reach the path delete an agent's socket and bind its own — and
-the gateway container has all of `/run/hive-agent` bind-mounted in.
-Dropping `o=w` removes that permission rather than qualifying it.
+nginx reaches all of `/run/hive-agent` (as a plain host path since the
+gateway moved out of its container; it used to be bind-mounted in, which
+was the same reach through a longer route). Dropping `o=w` removes that
+permission rather than qualifying it.
+
+⚠️ **The gateway leaving its container is a deliberate trade, recorded
+here so it is not mistaken for an oversight.** nginx and dnsmasq run on
+the host next to `hive-c0re` (see `docs/gateway.md`). What was given up
+is a *mount/pid* namespace — **not** a network one: that container ran
+with `privateNetwork = false` and shared the host's netns, so nginx was
+already binding host ports and already reaching `localhost` upstreams.
+The boundary bought no network isolation while costing a resolv.conf
+sync, a reload that had to cross the machine bus, and three bind mounts.
+🔑 It did cost one real thing, and the replacement is explicit: the
+privileged reload verb used to be scoped by `--machine=hive-gateway`,
+which could only ever reach into that one container. With no namespace
+to bound it, the unit name is hard-coded in `hive-priv` instead — see
+`PrivRequest::ReloadGatewayNginx`. **A caller cannot name the unit, so
+the verb cannot be steered at another service.**
 
 ⚠️ Contrast `/shared`, which *is* sticky world-writable (`1777`): it has
 many legitimate writers, so sticky is the best available answer there.