feat(caddy): serve Caddy root CA at /ca.crt for guest trust

Air-gap mirror uses tls internal; guests must trust Caddy's local CA. Serve it
at http://<domain>/ca.crt from a world-readable mount (the live pki dir is
root-only 0600). build-stack --up mirror|full exports the CA after bring-up;
landing page links it. Pre-creates the mount target to avoid a dir bind.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -16,3 +16,6 @@ www/
 landing/node_modules/
 landing/.astro/
 landing/dist/
+
+# exported Caddy CA (per-deploy artifact)
+caddy/caddy-root-ca.crt