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-forge/src/verbs/milestone.rs

@@ -41,8 +41,7 @@ pub fn run(client: &Client, args: Args) -> Result<()> {
     let repo = client.repo();
     match args.action.unwrap_or(Action::List) {
         Action::List => {
-            let v =
-                client.get_json(&format!("/repos/{repo}/milestones?state=open&limit=50"))?;
+            let v = client.get_json(&format!("/repos/{repo}/milestones?state=open&limit=50"))?;
             let trimmed: Vec<Value> = v
                 .as_array()
                 .map(|a| {