fix(atlas): UX polish — stage-06 plain nodes + shorter arrow labels (#7)

Stage 06 no longer renders bare in Plain: Build is now tolerant of a jobless/
absent workflow (keeps an authored plain placeholder instead of erroring), and
generated CI job nodes carry a plain hint. Shortened the two overlong transition
labels ("Order written, sealed, agent-ready" → "Sealed & agent-ready",
"Agents produced a change" → "Change proposed") so they stop clipping.

Build tolerance test-first. build/vet/lint(0)/test green; screenshot-verified.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/atlas/build_test.go

@@ -64,6 +64,20 @@ func TestDefault_HasPlainLayerAndTransitions(t *testing.T) {
 	}
 }
 
+func TestBuild_KeepsCIPlaceholderWhenNoJobs(t *testing.T) {
+	atlasJSON := []byte(`{"substrate":[],"stages":[
+	  {"no":"STAGE 06","title":"PR → CI","generate":"ci-jobs","nodes":[{"t":"Automated checks","plain":"checks run here"}]}
+	]}`)
+	// A workflow with no jobs block must NOT error and must keep the placeholder.
+	a, err := atlas.Build(atlasJSON, []byte("name: cd\n"))
+	if err != nil {
+		t.Fatalf("Build should tolerate a no-jobs workflow, got: %v", err)
+	}
+	if len(a.Stages[0].Nodes) != 1 || a.Stages[0].Nodes[0].Title != "Automated checks" {
+		t.Fatalf("placeholder not kept: %+v", a.Stages[0].Nodes)
+	}
+}
+
 func TestBuild_ErrorsOnBadAtlasJSON(t *testing.T) {
 	if _, err := atlas.Build([]byte("{not json"), []byte("jobs:\n  x:\n    a: 1\n")); err == nil {
 		t.Fatal("expected error on bad atlas JSON, got nil")