nix: openFirewall defaults false across forge/gateway/matrix (#651)

mara on #651: "Dont default openFirewall to true."

Flip the `openFirewall` default from `true` to `false` for all three
modules that expose host-side ports:

- `services.hyperhive.forge.openFirewall` (httpPort 3000 + sshPort 2222)
- `services.hyperhive.gateway.openFirewall` (port 80)
- `services.hyperhive.matrix.openFirewall` (httpPort 8008)

Rationale: secure-by-default. With shared host netns, the host +
every agent container reach these services via `localhost` regardless
of the firewall — the open only matters for access from outside the
host. Operators who want external reach now flip the bool explicitly:

    services.hyperhive.gateway.openFirewall = true;

Each description updated to explain the new default + when to flip
it (operator's browser, external git clients, federation announcement,
etc.). Behind a host-level reverse proxy that handles TLS, leave off.

Verified via `nix eval` on a clean stub config:
- forge openFirewall = false
- gateway openFirewall = false
- matrix openFirewall = false
- networking.firewall.allowedTCPPorts = [] (was: [80 2222 3000 8008])

Note: c0re's direct ports (7000/8000/8100-8999) are gated separately
via #621 on `gateway.enable` — that gate stays; this PR only touches
the per-module `openFirewall` knobs.

Closes #651.

nix/modules/hive-gateway.nix

@@ -82,11 +82,17 @@ in
 
     openFirewall = lib.mkOption {
       type = lib.types.bool;
-      default = true;
+      default = false;
+      example = true;
       description = ''
-        Open `port` in the host firewall. Off when the gateway should
-        only be reachable from inside the host (e.g. behind another
-        reverse proxy that handles TLS termination).
+        Open `port` in the host firewall. Off by default (#651,
+        secure-by-default). Flip to `true` to expose the gateway to
+        the operator's browser / external clients — required for any
+        out-of-host reach, since the agents themselves talk to
+        hive-c0re via the per-agent unix sockets and don't need the
+        nginx vhost. Leave off when running behind another reverse
+        proxy (e.g. caddy / traefik on the host) that handles TLS
+        termination + forwards to `port`.
       '';
     };