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.go

@@ -27,6 +27,7 @@ func Pipeline(p *model.Pipeline) string {
 	w("    classDef manual fill:#fc6d26,stroke:#e56b1f,color:#fff")
 	w("    classDef trigger fill:#6b4fbb,stroke:#5a3fa0,color:#fff")
 	w("    classDef delayed fill:#fca326,stroke:#d98a1e,color:#333")
+	w("    classDef on_failure fill:#d9534f,stroke:#c0392b,color:#fff")
 	w("")
 
 	// Collect visible (non-template) job names; sort for stable output.
@@ -149,6 +150,8 @@ func jobClass(job model.Job) string {
 		return "manual"
 	case "delayed":
 		return "delayed"
+	case "on_failure":
+		return "on_failure"
 	}
 	return "regular"
 }