ci: check docs against scripts

[locus313]

Summary

 .github/
 ├── scripts/
+│   └── check-docs.sh     # links, folder READMEs, header env vars vs README
 └── workflows/
+    └── docs.yml          # runs on .md/.sh/.bat changes (ci.yml ignores .md)

check-docs.sh
  for each tracked .md: every relative link resolves
  for each script: folder has README.md
  for each var in script header "Environment Variables": README mentions `VAR`

Follow-up to #38.

Evidence

• Before: nothing checked docs; monitor-aspnet.bat documented 5 of its 10 env vars (DOMAIN, NOTIFY_EMAIL, SMTP_FROM, STATE_DIR, MYSQL_PORT missing) and said SMTP values could only be set in the script.
  After: the first run of the check flagged all five, monitor-domain-hosting/README.md now lists them with defaults, and check-docs.sh prints Docs checks passed. shellcheck --severity=warning is clean.

Merge Danger

Door: two-way

Blast Radius: CI

New required-looking check on PRs touching docs or scripts. A header env var missing from a folder README now fails the build. Defaults and descriptions are not compared.