feat: wire ACME EAB into account registration + ZeroSSL auto-fetch

EAB credentials (KID + HMAC) were defined in the ACME connector config
but never wired into the acme.Account registration call. This fixes the
dead code and adds automatic EAB credential fetching for ZeroSSL — when
the directory URL is detected as ZeroSSL and no EAB credentials are
provided, certctl calls ZeroSSL's public API to get them automatically.

Changes:
- Wire EABKid/EABHmac into acme.Account.ExternalAccountBinding
- Add isZeroSSL() detection and fetchZeroSSLEAB() auto-fetch
- Add CERTCTL_ACME_EAB_KID/CERTCTL_ACME_EAB_HMAC env vars to main.go
- Add 13 ACME connector tests (config validation, EAB decode, ZeroSSL
  auto-EAB with mock servers, URL detection)
- Update docs: README, architecture, connectors, demo-advanced,
  testing-guide with EAB/auto-EAB documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/server/main.go

@@ -97,11 +97,14 @@ func main() {
 	localCA := local.New(localCAConfig, logger)
 	logger.Info("initialized Local CA issuer connector")
 
-	// Initialize ACME issuer connector (for Let's Encrypt, Sectigo, etc.)
+	// Initialize ACME issuer connector (for Let's Encrypt, ZeroSSL, Sectigo, Google Trust Services, etc.)
 	// Supports HTTP-01 (default), DNS-01 (for wildcards), and DNS-PERSIST-01 (standing record) challenge types.
+	// EAB (External Account Binding) required by ZeroSSL, Google Trust Services, SSL.com.
 	acmeConnector := acmeissuer.New(&acmeissuer.Config{
 		DirectoryURL:           os.Getenv("CERTCTL_ACME_DIRECTORY_URL"),
 		Email:                  os.Getenv("CERTCTL_ACME_EMAIL"),
+		EABKid:                 os.Getenv("CERTCTL_ACME_EAB_KID"),
+		EABHmac:                os.Getenv("CERTCTL_ACME_EAB_HMAC"),
 		ChallengeType:          os.Getenv("CERTCTL_ACME_CHALLENGE_TYPE"),
 		DNSPresentScript:       os.Getenv("CERTCTL_ACME_DNS_PRESENT_SCRIPT"),
 		DNSCleanUpScript:       os.Getenv("CERTCTL_ACME_DNS_CLEANUP_SCRIPT"),