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.

runbooks/mailcow-update.md

@@ -6,7 +6,7 @@ Path: `/opt/mail`
 
 ## Before
 
-1. Run [mailcow-health](mailcow-health.md) (or `./scripts/check-mx2.sh`). Record baseline.
+1. Run [mailcow-health](mailcow-health.md) (Ansible health report). Record baseline.
 2. Note config quirks in `mailcow.conf`, especially:
    - `SKIP_CLAMD=y` — after update, `clamd` may still be running until services are recreated per mailcow docs.
    - `SKIP_LETS_ENCRYPT`, ports, hostname — should stay `mx2.windy.me`.