Fix minimum Node 20 CLI compatibility

[rogerchappel]

Summary

• cover typegap --version against the package version
• avoid emitted JSON import-attribute syntax by using the existing TypeScript version module
• align engines, lockfile, README, and CI on Node 20.19, the minimum supported by the locked development toolchain

Commits

• test: cover CLI version output
• fix: avoid JSON import attributes in CLI
• docs: align Node 20 minimum with toolchain

Verification

Executed with Node.js v20.19.0:

npm ci
npm run release:check
git diff --check

The release gate passed 72 tests with coverage, built CLI help and fixture invocations, and the packed-installed CLI smoke check.