chore: add explicit node engine requirement

[isuretti]

Summary

Adds an explicit engines.node field to the root package.json to improve environment consistency across contributors and CI workflows.

Closes #2652

Changes

• added engines.node
• improved Node.js version visibility
• aligned repository tooling expectations

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[vercel]

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