ssh-hostcert + enrol-broker: fix defects surfaced by the F-02 bench

F-02/P-17 SSH host-cert flow was proven end-to-end on comet-beacon-54
(2026-07-11); these fix the correctness/scoping/cleanup defects the bench
surfaced:

- ssh-hostcert: derive the FQDN principal <hostname>.<domain> from the DHCP
  search domain at runtime (resolv.conf), alongside the short hostname, so an
  admin `@cert-authority *.<domain>` line accepts by-FQDN logins instead of
  "name is not a listed principal". Never baked (invariant #1). + tests.
- ssh-ca-bootstrap.sh: export the SSH host CA public key in OpenSSH wire format
  (ssh-keygen -i -m PKCS8), not the PEM/PKCS8 that `step crypto keypair` writes
  -- a PEM @cert-authority anchor is unparseable and silently falls back to TOFU.
- ssh-ca-bootstrap.sh: fail loudly (and verify enableSSHCA=true) if the
  provisioner SSH-enable step fails, instead of swallowing it and shipping a CA
  that 502s every runtime sign.
- dot1x-bench: point the co-located broker at https://localhost:8443, not the
  fleet dnsmasq name ca.fleet.lab (unresolvable from the broker's host netns ->
  "no such host" at sign time). localhost is in the CA cert SAN.
- serving: remove the mock-signer enrol-broker copy -- two host-networked
  brokers collide on 9090/9443 and the mock issued untrusted certs. The real
  broker lives with step-ca (dot1x-bench); nginx /enrol/ still proxies to it on
  loopback.
- enrol-broker: retire the HTML admin console; the fleet-manager passkey SPA now
  drives the mTLS JSON API (/pending, /approve|deny). Removes a weaker,
  client-cert-only parallel human-approve path.

Design-doc bench note deferred: docs/ssh-host-certificate-design.md lives on the
secure-ntp branch (fbb8100), not here -- to be added there / on reconcile.

files/ssh-hostcert/main.go

@@ -77,9 +77,15 @@ func main() {
 	if err != nil {
 		log.Fatalf("read host key %s: %v", *hostKey, err)
 	}
-	princ := splitPrincipals(*principals)
+	// Principals are the names admins connect by. Default to this host's
+	// (DHCP-assigned) hostname AND its FQDN <hostname>.<domain> derived from the
+	// DHCP search domain at runtime (never baked — invariant #1); any explicit
+	// -principals are added on top. Without the FQDN a `@cert-authority *.<domain>`
+	// line rejects the cert with "name is not a listed principal" on FQDN logins
+	// (bench-confirmed on comet-beacon-54, 2026-07-11).
+	princ := appendUnique(defaultPrincipals(), splitPrincipals(*principals)...)
 	if len(princ) == 0 {
-		princ = defaultPrincipals()
+		princ = []string{"localhost"}
 	}
 	keyID := ""
 	if id.cert.Leaf != nil {
@@ -175,12 +181,58 @@ func splitPrincipals(s string) []string {
 	return out
 }
 
+// defaultPrincipals returns the names admins connect to this host by: the
+// (DHCP-assigned) hostname and, when a DHCP search domain is known, the FQDN
+// <hostname>.<domain>. The FQDN is derived at runtime (never baked — invariant
+// #1), mirroring how the NTS dispatcher derives ntp.<domain>: an admin's
+// `@cert-authority *.<domain>` line only matches the host if the FQDN is a cert
+// principal, so a short-name-only cert fails FQDN logins.
 func defaultPrincipals() []string {
 	h, err := os.Hostname()
 	if err != nil || h == "" {
-		return []string{"localhost"}
+		return nil
 	}
-	return []string{h}
+	out := []string{h}
+	if !strings.Contains(h, ".") {
+		if dom := dhcpDomain(); dom != "" {
+			out = append(out, h+"."+dom)
+		}
+	}
+	return out
+}
+
+// resolvConf is the file dhcpDomain reads (a var so tests can point elsewhere).
+var resolvConf = "/etc/resolv.conf"
+
+// dhcpDomain returns the first DHCP-provided search/domain from resolv.conf
+// (NetworkManager writes it from DHCP option 15/119). Empty if none is set.
+func dhcpDomain() string {
+	b, err := os.ReadFile(resolvConf)
+	if err != nil {
+		return ""
+	}
+	for ln := range strings.SplitSeq(string(b), "\n") {
+		f := strings.Fields(ln)
+		if len(f) >= 2 && (f[0] == "search" || f[0] == "domain") {
+			return f[1]
+		}
+	}
+	return ""
+}
+
+// appendUnique appends to base each of extra not already present (order-preserving).
+func appendUnique(base []string, extra ...string) []string {
+	seen := make(map[string]bool, len(base))
+	for _, s := range base {
+		seen[s] = true
+	}
+	for _, s := range extra {
+		if s != "" && !seen[s] {
+			seen[s] = true
+			base = append(base, s)
+		}
+	}
+	return base
 }
 
 func envOr(k, def string) string {