fix(#775): enforce Node.js >= 20 on startup

[snowrugar-beep]

Closes #775
Closes #776
Closes #777
Closes #778

Bootstrap now enforces Node.js >= 20 before importing any module. Several transitive deps (e.g. @nestjs/* v11) require Node 20+, but package.json's engines field only declared >= 18. The previous check was permissive at 18; this PR tightens it to 20, adds a Number.isNaN guard, and links users to the upgrade page.

• Bumped threshold from < 18 to < 20 (matches issue spec)
• Added Number.isNaN guard for malformed process.versions.node
• Error message points users to https://nodejs.org/ to upgrade
• Kept existing logger.error + process.exit(1) pattern for consistency
• tsc clean, lint clean on main.ts, full test suite unaffected

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[nanaf6203-bit]

LGTM