Require Node.js 20

[scott-rc]

This bumps our minimum supported version of Node.js from 18 -> 20.

Node.js 18 reached End-of-Life (EOL) on 2025-04-30:

• https://github.com/nodejs/release#end-of-life-releases
• https://nodejs.org/en/blog/announcements/node-18-eol-support

---

Note

Raise minimum Node.js to 20 and update CI, Nix, and dependencies to match.

• BREAKING:
  • Require Node.js >=20 via package.json#engines and add a changeset (.changeset/node-20.md).
• CI/CD:
  • Switch GitHub Actions workflows (.github/workflows/ci.yml, cd.yml) to Node 20 for lint, build, test, and release.
• Dev Environment (Nix):
  • Use nodejs_20 in nix/flake.nix and update nix/flake.lock inputs.
• Dependencies/Tooling:
  • Bump @types/node to ^20 and refresh pnpm-lock.yaml accordingly.
  • Tweak version script to run prettier --write CHANGELOG.md.
  • Clean up Dependabot config: keep @types/node major updates ignored; remove other ignores.

^{Written by Cursor Bugbot for commit 075a7ab. This will update automatically on new commits. Configure here.}

[changeset-bot]

🦋 Changeset detected

Latest commit: 075a7ab

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
ggt	Major

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