image/kernel pull: heartbeat dots so slow pulls look alive

Bundle downloads can take 20–60s on a typical connection and the
CLI was going silent between "resolving daemon" and the final image
summary. Users wondered whether banger had wedged.

New `withHeartbeat` helper wraps an RPC call with a dot-every-2s
ticker on stderr. No-op when stderr isn't a terminal, so piped or
scripted invocations stay quiet. Wired into `image pull` and `kernel
pull`, the two commands that actually download bytes.

Example:

    $ banger image pull debian-bookworm
    [image pull] ..........
    id  name             managed  ...

Tests cover the non-TTY short-circuit and error propagation.

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

internal/cli/cli_test.go

@@ -574,6 +574,33 @@ func TestVMRunProgressRendererSuppressesDuplicateLines(t *testing.T) {
 	}
 }
 
+func TestWithHeartbeatNoOpForNonTTY(t *testing.T) {
+	var buf bytes.Buffer
+	called := false
+	err := withHeartbeat(&buf, "image pull", func() error {
+		called = true
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("withHeartbeat: %v", err)
+	}
+	if !called {
+		t.Fatal("fn should have been called")
+	}
+	if buf.Len() != 0 {
+		t.Fatalf("stderr = %q, want empty for non-TTY", buf.String())
+	}
+}
+
+func TestWithHeartbeatPropagatesError(t *testing.T) {
+	sentinel := errors.New("boom")
+	var buf bytes.Buffer
+	err := withHeartbeat(&buf, "image pull", func() error { return sentinel })
+	if !errors.Is(err, sentinel) {
+		t.Fatalf("withHeartbeat error = %v, want %v", err, sentinel)
+	}
+}
+
 func TestVMSetParamsFromFlagsConflict(t *testing.T) {
 	if _, err := vmSetParamsFromFlags("devbox", -1, -1, "", true, true); err == nil {
 		t.Fatal("expected nat conflict error")