feat(mgmtplane): break-glass privsep-dir watchdog + mgmt_plane health (TASK G1) — v0.71.0

Prerequisite for felhom-sshd (H1). Closes the SPIKE-felhom-sshd §8 lockout: a
second sshd's RuntimeDirectory=sshd removed the SHARED /run/sshd privsep dir and
took stock sshd on :22 down (sessions reset after KEXINIT).

Host artifacts (configs/, installed by felhom-host-install):
- felhom-privsep.tmpfiles: layer 1, boot-persistent /run/sshd owned by no unit
- felhom-mgmt-watchdog.sh/.service/.timer: layer 2, AGENT-INDEPENDENT ~60s heal
  (stat-first recreate + reset-failed sshd only if failed + heal-marker); never
  RuntimeDirectory=, never restarts stock sshd, never touches a healthy dir.

Go (internal/mgmtplane): read-only Reporter → additive omitempty mgmt_plane
heartbeat stanza (privsep_dir_ok/sshd_reachable/healed_recently/privsep_healed_at),
wired via Collector.SetMgmtPlaneReporter. Non-hollow tests + red-proofs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

internal/hub/collect_mgmtplane_test.go

@@ -0,0 +1,41 @@
+package hub
+
+import (
+	"context"
+	"testing"
+)
+
+// fakeMgmtPlane is a MgmtPlaneReporter returning a fixed stanza (or nil).
+type fakeMgmtPlane struct{ st *MgmtPlaneStatus }
+
+func (f fakeMgmtPlane) MgmtPlaneStatus(context.Context) *MgmtPlaneStatus { return f.st }
+
+func TestCollect_MgmtPlaneOmittedWhenReporterNil(t *testing.T) {
+	px := &fakePx{node: "n", ns: newTestNodeStatus()}
+	c := NewCollector(px, fakeProber{status: "active"}, fakeObserver{}, nil, nil, nil, "h", "0.71.0", quietLogger())
+	r, err := c.Collect(context.Background())
+	if err != nil {
+		t.Fatalf("Collect: %v", err)
+	}
+	if r.MgmtPlane != nil {
+		t.Fatalf("no reporter wired → mgmt_plane must be omitted (nil), got %+v", r.MgmtPlane)
+	}
+}
+
+func TestCollect_MgmtPlanePopulatedWhenWired(t *testing.T) {
+	px := &fakePx{node: "n", ns: newTestNodeStatus()}
+	c := NewCollector(px, fakeProber{status: "active"}, fakeObserver{}, nil, nil, nil, "h", "0.71.0", quietLogger())
+	c.SetMgmtPlaneReporter(fakeMgmtPlane{st: &MgmtPlaneStatus{
+		PrivsepDirOK: true, SshdReachable: true, HealedRecently: true, PrivsepHealedAt: "2026-07-05T16:42:17Z",
+	}})
+	r, err := c.Collect(context.Background())
+	if err != nil {
+		t.Fatalf("Collect: %v", err)
+	}
+	if r.MgmtPlane == nil {
+		t.Fatal("wired reporter → mgmt_plane must be present")
+	}
+	if !r.MgmtPlane.HealedRecently || r.MgmtPlane.PrivsepHealedAt != "2026-07-05T16:42:17Z" {
+		t.Fatalf("mgmt_plane not carried through: %+v", r.MgmtPlane)
+	}
+}