chore(docs,nix): fix stale 'localhost via shared netns' claims in forge/matrix

Agents in private netns can't reach host loopback directly; they
use forge.<domain> and matrix.<domain> via the gateway. Update four
locations that still claimed direct loopback access:

- hive-forge.nix: module header + openFirewall description
- hive-matrix.nix: openFirewall description
- docs/matrix.md: container design bullet + openFirewall section

docs/matrix.md

@@ -17,10 +17,9 @@ Same shape as [`gateway.md::hive-forge container shape`](gateway.md):
   operator already runs on the host — separate systemd namespace,
   separate state dir.
 - Container shares the host network namespace
-  (`privateNetwork = false`) so agents reach tuwunel at
-  `http://localhost:<httpPort>` without extra plumbing — the
-  nixos-container is here for state + systemd-unit isolation, not
-  network isolation.
+  (`privateNetwork = false`) for state + systemd-unit isolation. Agents
+  reach the homeserver at `matrix.<domain>` via the gateway (agents
+  run in private netns and can't access host loopback directly).
 - Persistent state at
   `/var/lib/nixos-containers/hive-matrix/var/lib/matrix-tuwunel/`
   survives container restart / host reboot. To wipe, destroy the
@@ -52,9 +51,9 @@ sub-domain from the user-facing identifier.
 
 ## Default-closed firewall
 
-`openFirewall` defaults to `false` (secure-by-default): the
-homeserver is reachable from the host + every agent container via
-loopback either way (shared netns), so the firewall hole only
+`openFirewall` defaults to `false` (secure-by-default): the host
+reaches the homeserver on loopback, and agent containers reach it
+at `matrix.<domain>` via the gateway — so the firewall hole 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.