feat(gateway): hivectl gateway user management + fix htpasswdFile assertion

Add `hivectl gateway {create-user,delete-user,list-users}` subcommands for
managing htpasswd files used by gateway Basic auth. Pure Rust bcrypt
(cost 12, $2y$ prefix nginx accepts). No external htpasswd binary required.

Also fix the NixOS module assertion: `cfg.auth ? htpasswdFile` is always
true in the module system (declared options always exist as keys); switch
to `nullOr path; default = null` + `!= null` check so the assertion
actually fires with a useful error when enable=true but no file is set.
Guard bind-mount and nginx config against null to prevent eval errors.

Update docs/gateway.md to show hivectl commands instead of raw htpasswd.

hive-c0re/src/scheduled_prompts_worker.rs

@@ -101,7 +101,10 @@ fn fire_schedule(coord: &Arc<Coordinator>, schedule: &Schedule, now: i64) {
         // scheduled prompt from stacking up when an agent is slow or
         // briefly offline, while still allowing distinct scheduled
         // messages (different body) to enqueue independently.
-        match coord.broker.has_pending_with_body(target, "scheduled", &schedule.body) {
+        match coord
+            .broker
+            .has_pending_with_body(target, "scheduled", &schedule.body)
+        {
             Ok(true) => {
                 tracing::debug!(
                     schedule = schedule.id,
@@ -384,4 +387,3 @@ async fn known_agents_async() -> std::collections::HashSet<String> {
     }
     out
 }
-