docs: improve vgraph skill guidance

[fangsmile]

Summary

• Narrow VGraph skill activation wording to VGraph-specific docs/demos/migration guidance
• Add explicit Do NOT load guidance for reference routing
• Prefer TreeGraph native collapse/expand/toggleCollapse for hierarchy demos
• Replace unsupported dragCanvas eventType example with source-backed shouldTrigger guidance
• Use try/finally for layout batch mutation examples

Verification

• git diff --check
• git push pre-push hook: rush test --only tag:package
  • @visactor/vgraph: 131 passed, 131 total
  • @visactor/react-vgraph: 1 passed, 1 total
  • @visactor/react-vgraph-ui: completed successfully