refactor(gateway): make the gateway unconditional — remove gateway.enable

The gateway container starts alongside every hyperhive deployment, so
gating it behind a separate enable flag was a footgun: an operator who
set it false lost the only thing exposed to the outside while the
agent containers kept running. Re-gate the gateway config on the
top-level services.hyperhive.enable instead.

- hive-gateway.nix: drop the gateway.enable mkOption; gate the config
  block on config.services.hyperhive.enable.
- hive-forge.nix: behindGateway now defaults to services.hyperhive.enable;
  remove the behindGateway-requires-gateway assertion (now vacuous).
- hive-network.nix: remove both gateway.enable assertions (vacuous).
- hive-c0re.nix: drop the firewall.allowedTCPPortRanges 8100-8999
  fallback that opened agent ports when the gateway was off (the
  gateway is now the sole entry point); HIVE_GATEWAY_ENABLED is always
  set since the gateway always runs.
- nix/docs/default.nix: remove the gateway.enable = mkForce false stub
  (would be an eval error against the removed option; the gateway is
  already re-gated on hyperhive.enable, which docs force false).
- hive-matrix.nix, dashboard.rs: comment/prose updates only.

BREAKING: operators relying on services.hyperhive.gateway.enable = false
to suppress the gateway must instead point their own reverse proxy at
the gateway's port. NixOS errors clearly on the now-unknown option.

nix/docs/default.nix

@@ -31,7 +31,6 @@ let
           services.hyperhive.enable = lib.mkForce false;
           services.hyperhive.forge.enable = lib.mkForce false;
           services.hyperhive.matrix.enable = lib.mkForce false;
-          services.hyperhive.gateway.enable = lib.mkForce false;
         }
       )
     ];