feat(base): add the 'mesh' concern tag (NetBird agent, ADR-016)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/tags.yml

@@ -20,6 +20,7 @@ concerns:
   - config       # render templated config/compose files to disk — no restart
   - deploy       # bring services up / restart (compose up -d)
   - proxy        # reverse-proxy + TLS registration (Caddy routes, Authentik)
+  - mesh         # NetBird agent enrollment (ADR-016)
 
 # Ansible built-in special tags. Narrow use only:
 #   always — cheap preflight assertions (run regardless of --tags)