Skip empty tonie folders instead of pruning all chapters

Previously, an accidentally empty tonie folder combined with pruning
(the default) would delete every existing chapter on the tonie. Now:

- syncer.BuildPlan detects an empty local folder and returns a
  Plan{Skipped: true, SkipReason: ...} instead of computing a diff
- syncer.ApplyPlan is a no-op for a skipped plan (defense in depth)
- Plan.NeedsChanges() reports false for skipped plans
- cmd/toni-sync sync prints "Skipped: <reason>" for such folders and
  moves on, regardless of --no-prune
- Added a test covering the skip behavior end-to-end (BuildPlan +
  ApplyPlan no-op)
- README documents the safety behavior

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/syncer/syncer_test.go

@@ -129,6 +129,32 @@ func TestBuildPlanNoPruneKeepsStaleChapters(t *testing.T) {
 	}
 }
 
+func TestBuildPlanSkipsEmptyFolder(t *testing.T) {
+	dir := t.TempDir() // no audio files
+	tonie := makeTonie([]tonieapi.Chapter{{ID: "c1", Title: "existing chapter"}})
+	client := &fakeClient{tonieSequence: []tonieapi.CreativeTonie{tonie}}
+
+	m := Target{TonieID: "t1", Folder: dir, Prune: true}
+	plan, err := BuildPlan(client, m)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !plan.Skipped {
+		t.Fatal("expected plan to be skipped for empty folder")
+	}
+	if plan.NeedsChanges() {
+		t.Fatal("expected NeedsChanges to be false for a skipped plan")
+	}
+
+	if err := ApplyPlan(client, m, plan); err != nil {
+		t.Fatal(err)
+	}
+	if len(client.sortedTitles) != 0 || len(client.uploadedTitles) != 0 {
+		t.Fatalf("expected ApplyPlan to be a no-op for a skipped plan, got uploads=%v sorted=%v", client.uploadedTitles, client.sortedTitles)
+	}
+}
+
 func TestApplyPlanUploadsAndSkipsReorderWhenAlreadyMatching(t *testing.T) {
 	dir := setupFolder(t)
 	before := makeTonie([]tonieapi.Chapter{{ID: "c1", Title: "01 - First"}})