fix(#1061): suppress tuwunel default displayname suffix

Tuwunel 1.6.1 appends " 💕" to every new user's display name via the
new_user_displayname_suffix config option. Set it to empty string so
agent and operator matrix accounts get clean names.

Existing users (atlas, mara, root) need a one-time display name reset.
Atlas's was already corrected via the client API.

nix/modules/hive-matrix.nix

@@ -359,6 +359,9 @@ in
               registration_token_file = "/run/credentials/tuwunel.service/registration_token";
               # E2EE disabled in initial rollout; tracked in the issue tracker.
               allow_encryption = false;
+              # Tuwunel's default suffix is " 💕" — suppress it so agent
+              # display names are clean (just the agent name, no emoji).
+              new_user_displayname_suffix = "";
             };
           };
           # Keeps DynamicUser=true + PrivateUsers=true intact — no