feat: Dashboard workflow graph visualization with React Flow (#198)

Phase 1: API + static graph rendering

Backend:
- GET /workflows/:name now returns descriptor (with graph) from bundle YAML
- Graceful fallback to null if YAML missing/invalid

Frontend:
- New workflow-graph/ component module (7 files)
- React Flow + dagre auto-layout (TB direction)
- Custom nodes: RoleNode (rounded rect) + TerminalNode (circle for START/END)
- Custom edges: dashed for FALLBACK, solid with label for conditions
- Self-loop edges supported (e.g. coder → coder)
- Node states: default/completed/active with color-coded borders
- Active node pulse animation
- Collapsible graph panel (300px) above thread records
- Dark theme using existing CSS variables

Integration:
- ThreadDetail extracts workflow name → fetches descriptor → computes node states → renders graph
- Node states derived from ThreadRecord[] (completed/active/default)

packages/workflow-dashboard/package.json

@@ -9,6 +9,8 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@dagrejs/dagre": "^3.0.0",
+    "@xyflow/react": "^12.10.2",
     "react": "^19.2.6",
     "react-dom": "^19.2.6",
     "react-markdown": "^10.1.0",