feat(graph): pipeline graph improvements — on_failure, tooltips, bus-bar connectors, skipped colouring, HTML and Mermaid output

- when: on_failure visual distinction: red circle (#d9534f), X-mark icon, dashed
  chip border, and Mermaid classDef; legend entry added
- Job tooltip / detail panel: each chip wrapped in <g data-job="…"><title>…</title>
  <desc>…</desc> so SVG viewers and the HTML sidebar show stage, when, image, needs
- Multi-job connector accuracy: classic mode now uses a bus-bar pattern (vertical
  rail at midpoint + per-job stubs) instead of one center-to-center line per stage pair
- Blocked/skipped state colouring: RenderPipeline and pipelineSVG accept
  *cicontext.Context; skipped jobs rendered in grey (#868686) with dimmed text
- Interactive HTML output (--format html): self-contained .html with inline SVG,
  mouse-wheel zoom, drag-to-pan, double-click reset, and a click-to-open sidebar
- Mermaid pipeline output (--format mermaid): prints existing Pipeline() flowchart
  to stdout; suitable for mermaid.live or Markdown embedding
- imageString() helper to extract image name from string or map form
- 100% statement coverage maintained; 809 tests pass

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/graph/pipeline_test.go

@@ -105,10 +105,23 @@ func TestSanitizeID(t *testing.T) {
 func TestJobClass(t *testing.T) {
 	if jobClass(model.Job{When: "manual"}) != "manual" { t.Error("manual") }
 	if jobClass(model.Job{When: "delayed"}) != "delayed" { t.Error("delayed") }
+	if jobClass(model.Job{When: "on_failure"}) != "on_failure" { t.Error("on_failure") }
 	if jobClass(model.Job{Trigger: "x"}) != "trigger" { t.Error("trigger") }
 	if jobClass(model.Job{}) != "regular" { t.Error("regular") }
 }
 
+func TestPipeline_OnFailureClass(t *testing.T) {
+	p := &model.Pipeline{
+		Stages: []string{"cleanup"},
+		Jobs: map[string]model.Job{
+			"cleanup-job": {Name: "cleanup-job", Stage: "cleanup", When: "on_failure"},
+		},
+	}
+	out := Pipeline(p)
+	if !strings.Contains(out, "on_failure") { t.Error("expected on_failure class") }
+	if !strings.Contains(out, "#d9534f") { t.Error("expected on_failure color in classDef") }
+}
+
 // ── needsJobName ──────────────────────────────────────────────────────────────
 
 func TestNeedsJobName(t *testing.T) {