feat(spawn): share host spawn-pi mesh + models.json with spawned containers

- bind $LVMH_HOST_PI_RUNTIME_DIR/spawn-pi -> /root/.pi/spawn-pi (rw) so
  container pi's join the host pi's node mesh (spawn_pi/list_pi_nodes/
  send_pi_message now reach across the host/container boundary)
- bind host models.json (when present) over the stale image-baked
  fallback so newly added catalog models resolve in spawns

daemon/docker_test.go

@@ -519,6 +519,64 @@ func TestSpawnerSecretsBinds(t *testing.T) {
 	}
 }
 
+func TestWorkerBindsHostModelsJSON(t *testing.T) {
+	useFakeGit(t, fakeGitModeOK)
+	f := newFakeDocker()
+	sp, _ := newTestSpawner(t, f)
+	agent := t.TempDir()
+	if err := os.WriteFile(filepath.Join(agent, "models.json"), []byte(`{}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv(envHostPiAgentDir, agent)
+
+	res, err := sp.Start(context.Background(), "group/project", "", "", false)
+	if err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	waitJobState(t, sp, res.SessionID, stateRunning)
+	creates := f.createsByName("lvmh-agent-")
+	binds := creates[0].HostConfig.Binds
+	want := agent + "/models.json:/root/.pi/agent/models.json:ro"
+	ok := false
+	for _, b := range binds {
+		if b == want {
+			ok = true
+		}
+	}
+	if !ok {
+		t.Fatalf("models.json bind missing from %v", binds)
+	}
+}
+
+func TestWorkerBindsSpawnPiMesh(t *testing.T) {
+	useFakeGit(t, fakeGitModeOK)
+	f := newFakeDocker()
+	sp, _ := newTestSpawner(t, f)
+	runtimeDir := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(runtimeDir, "spawn-pi"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv(envHostPiRuntimeDir, runtimeDir)
+
+	res, err := sp.Start(context.Background(), "group/project", "", "", false)
+	if err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	waitJobState(t, sp, res.SessionID, stateRunning)
+	creates := f.createsByName("lvmh-agent-")
+	binds := creates[0].HostConfig.Binds
+	want := filepath.Join(runtimeDir, "spawn-pi") + ":/root/.pi/spawn-pi"
+	ok := false
+	for _, b := range binds {
+		if b == want {
+			ok = true
+		}
+	}
+	if !ok {
+		t.Fatalf("spawn-pi mesh bind missing from %v", binds)
+	}
+}
+
 func TestWorkerBindsCaches(t *testing.T) {
 	useFakeGit(t, fakeGitModeOK)
 	f := newFakeDocker()