fix: align Node.js engine requirement with runtime dependencies

[m1ngsama]

What this PR changes

• Updates package.json engine from >=16.0.0 to >=20.12.0.
• Updates README runtime requirement to match.
• Adds .nvmrc with 20.12.0 for consistent local development.

Why

The current dependency graph (notably modern @inquirer/*) requires Node 20+, while the project metadata/docs still state Node 16. This mismatch causes avoidable installation and runtime confusion.

Validation

• Ran npm run build successfully on Node v25.6.1.

Related

Closes #10

[m1ngsama]

Closing as superseded.

The core fix from this PR (engine >=20.12.0) is already in main via the v1.0.20 refactor commit. The CI workflow improvements (node matrix [20, 22], EOTP diagnostics in publish.yml) were also already present in main at the time of that commit.

Cherry-picked additions from this branch:

• .nvmrc added directly to main ✅
• .github/workflows/release.yml (auto-tag on PR merge) added to main ✅

Closes #10