fix(m3-hermes): stable Matrix device + DNS pin for homeserver

Root cause chain of recurring Matrix outages (2026-08-16):

1. DNS on m3-hermes resolves exclusively via the Netbird-managed
   resolver. When the Netbird nameserver group is unreachable
   (observed 14:03-15:24), matrix.m3ta.dev fails to resolve.
2. A gateway restart during that window fails the Matrix login, and
   without MATRIX_ACCESS_TOKEN the gateway drops Matrix permanently
   from its reconnect queue ('no bot credential on queued config').
   Recovery happens only on the next restart - with working DNS.
3. The stable-token setup from 2026-08-14 was wiped because it was
   only written to the regenerated .env, not to the agenix secret.

Changes:
- Pin matrix.m3ta.dev to the m3-atlas public IP (TLS termination)
  so the Matrix connection survives Netbird DNS outages.
- Declare MATRIX_DEVICE_ID=HERMES01 in the module environment so the
  device id survives .env regeneration. The matching
  MATRIX_ACCESS_TOKEN must be added to secrets/hermes-env.age
  (separate manual step: agenix edit + re-encrypt).

hosts/m3-hermes/configuration.nix

@@ -13,6 +13,18 @@
   networking.hostName = "m3-hermes";
   networking.hostId = "a1b2c3d4"; # TODO: Generate unique hostId
   networking.networkmanager.enable = true;
+
+  # Matrix homeserver pin: DNS on this host resolves exclusively via the
+  # Netbird-managed resolver (see /etc/resolv.conf → wt0). When the Netbird
+  # nameserver group is unreachable, matrix.m3ta.dev fails to resolve and the
+  # gateway cannot log in (observed 2026-08-16). Pinning the public IP of
+  # m3-atlas (TLS termination for matrix.m3ta.dev) makes the Matrix connection
+  # independent of Netbird DNS health. Update here if the m3-atlas public IP
+  # ever changes.
+  networking.hosts = {
+    "152.53.85.162" = ["matrix.m3ta.dev"];
+  };
+
   time.timeZone = "Europe/Berlin";
   i18n.defaultLocale = "en_US.UTF-8";