Agent host check: run every step, and say so

The check went to the host on stdin, and the first docker compose exec read
the rest of it: the memory and firewall checks never ran, and it passed.
It is now its own file, sent as an argument, and ends with "all checks
passed" only when every check has run. Tests fake the host with a docker
that reads stdin.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

deploy/agent-host/README.md

@@ -14,6 +14,8 @@ later (M6.1).
   `/opt/stacks/silverbullet` and the firewall, and starts it. Safe to run again.
 - `nisse-firewall`: only the operator's Caddy may reach SilverBullet's port
   (Docker's `DOCKER-USER` chain, re-applied whenever Docker starts).
+- `check.sh`: run on the host by `scripts/agent-host.sh check`; says "all
+  checks passed" only once every check has run.
 - `caddy/vault.dongar.net.caddy`: the site for the operator's Caddy on
   `docker.garrett-hall.com`. HTTPS from Caddy's usual certificate, LAN only.