docs(gateway): describe what is, not what changed

Per review: docs represent current state. Every "used to" / "no longer"
clause this branch introduced is gone — including the History section in
network.md, which was a whole subsection about a sync mechanism that
doesn't exist.

Where the removed clause was carrying a real constraint, the constraint
stays and is stated in the present tense instead of as a delta: nothing
narrows what the gateway's nginx can reach except the directory
permissions in front of a socket, and nothing bounds `ReloadGatewayNginx`
except the hard-coded unit name. Those read as rules now rather than as
the story of how they came to be rules.

hive-c0re/src/gateway_nginx.rs

@@ -167,16 +167,12 @@ fn render(names: &[String], frontend_dir: Option<&str>) -> String {
 /// After a successful write, triggers the appropriate nginx action via
 /// `hive-priv` (hive-c0re runs unprivileged and cannot act on a system
 /// unit): reload when nginx is active, reset-failed+start when in a
-/// failed state, plain start otherwise. Writer and nginx are now on the
-/// same machine, so this is a plain unit action rather than the old
-/// `systemd-run --machine=hive-gateway` hop across the container
-/// boundary. It stays an explicit trigger rather than a systemd path
-/// unit watching the file. A path unit would now *work* — `IN_MOVED_TO`
-/// (fired by the atomic rename) failed to propagate across the nspawn
-/// mount-namespace boundary, and that boundary is gone — but it is still
-/// not wanted: the write already knows it changed something, and a
-/// watcher turns one causal edge into a race with the writer's own
-/// rename (see `docs/gateway.md`).
+/// failed state, plain start otherwise. Writer and nginx are on the same
+/// machine, so it is a plain unit action with no machine-bus hop.
+/// It stays an explicit trigger rather than a systemd path
+/// unit watching the file: the write already knows it changed something,
+/// and a watcher would turn one causal edge into a race with the writer's
+/// own rename (see `docs/gateway.md`).
 ///
 /// The priv call is best-effort — a failed sync is logged but not fatal.
 /// `reload_if_pending` retries on the next `spawn_poll` tick so a