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-matrix.nix

@@ -98,12 +98,16 @@ in
 
     openFirewall = lib.mkOption {
       type = lib.types.bool;
-      default = true;
+      default = false;
+      example = true;
       description = ''
-        Open `httpPort` in the host firewall. Off when the
-        homeserver should only be reachable from inside the host
-        (e.g. while bringing the integration up before announcing
-        it to other hives).
+        Open `httpPort` in the host firewall. Off by default (#651,
+        secure-by-default): the homeserver is reachable from the
+        host + every agent container via `localhost` either way
+        (shared netns), so the firewall open only matters for
+        access from outside the host. Flip to `true` when announcing
+        the homeserver to other hives or when an external matrix
+        client needs to reach the client-server API directly.
 
         Note: federation (the matrix-spec well-known port 8448) is
         intentionally not opened here. tuwunel serves the federation