docs: onboard AdGuard Home to Ansible; retire check-mx2; migrate plan to Linear

- Add dns.windy.lan to Ansible inventory (adguardhome group/profile) with a
  read-only healthcheck template, systemd timer, and on-demand report parity
  with the other active hosts; verified green on the host.
- Add synapse.chans.xyz to the AGENTS.md active-host quick map.
- Remove plans/; migrate decisions/verification to a Linear vps project doc and
  file the remaining gaps as W1N-46..49 with blocking edges.
- Retire scripts/check-mx2.sh; point mailcow health/update runbooks and the
  mx2 host file at the Ansible health report.
- Trim us2 stub software to verified running services; note dormant dirs.
- Normalize repo docs to English; drop the non-existent CONTEXT.md/ADR claim
  and remove docs/agents/domain.md.

ansible/inventory/hosts.yml

@@ -56,6 +56,15 @@ all:
           service_reconcile_services:
             wireguard:
               compose_args: [--no-deps, --force-recreate]
+        dns_windy_lan:
+          ansible_host: 192.168.66.36
+          ansible_host_ipv4: 192.168.66.36
+          service_role: adguardhome
+          compose_project_dir: /opt/adguardhome
+          healthcheck_profile: adguardhome
+          service_reconcile_services:
+            adguardhome:
+              compose_args: [--no-deps, --force-recreate]
     mailcow:
       hosts:
         mx2:
@@ -68,12 +77,16 @@ all:
     wireguard:
       hosts:
         us4:
+    adguardhome:
+      hosts:
+        dns_windy_lan:
     docker_hosts:
       children:
         mailcow:
         vaultwarden:
         powerdns:
         wireguard:
+        adguardhome:
     # Matrix is a dedicated K3s node and intentionally remains outside the
     # Docker-oriented managed group.
     k3s_servers: