Add root CLI bootstrap contract leaf

[bingran-you]

Summary

• add a product-surface leaf for root CLI flag contracts and launch-time bootstrap overrides
• cover headless mode gates, explicit startup injections, version-sensitive --bare behavior, and startup file downloads via --file
• link the new leaf from product-surface/NODE.md

Verification

• npx -p first-tree first-tree verify
• git diff --check