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/workers/agent_sockets.rs

@@ -14,10 +14,11 @@ use std::path::{Path, PathBuf};
 use anyhow::{Context, Result};
 
 /// Host-side parent directory holding per-agent socket subdirs. The
-/// gateway container bind-mounts this whole tree (read-only) so it
-/// can `proxy_pass` to any agent. Each agent's container bind-mounts
-/// only its own `<name>/` subdir — agents can only access their own
-/// sockets. The literal lives in `hive-host-sock` (shared with
+/// gateway's nginx runs on the host and reads this whole tree, so it
+/// can `proxy_pass` to any agent — it used to get there through a
+/// read-only bind-mount of the same tree. Each agent's container
+/// bind-mounts only its own `<name>/` subdir, which is still what stops
+/// one agent reaching another's socket. The literal lives in `hive-host-sock` (shared with
 /// `hivectl`); re-exported here under the name this module's consumers
 /// have always used.
 pub use hive_host_sock::AGENT_SOCKET_DIR;
@@ -166,7 +167,7 @@ pub fn write(names: &[String]) -> Result<()> {
 ///
 /// Also calls `gateway_nginx::reload_if_pending` on every tick to
 /// retry a gateway nginx reload that may have failed on the previous
-/// tick (e.g. gateway container temporarily down). This recovers
+/// tick (e.g. gateway nginx temporarily down). This recovers
 /// gateway routing without needing a manual gateway restart.
 ///
 /// Mirrors the spawn-loop shape used by `crash_watch`,