docs(readme): correct Node.js requirement — 20+, not 18+, to match engines pin (#190)

The Requirements section said "Node.js 18+ (tested on 20 and 22)"
but `package.json` declares `"engines": { "node": ">=20.0.0" }`,
the README badge already says "Node.js 20+", and the CI matrix
only tests Node 20 and 22. A contributor on Node 18 reading just
the Requirements line would `npm install` and get an engines-
mismatch warning that contradicts the docs.

One-line fix bringing the README in line with the engines pin
+ CI + badge. No other content change.

Co-authored-by: Aaron K. Clark <akclark@thenetwerk.net>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

README.md

@@ -91,7 +91,7 @@ node -e "require('fs').writeFileSync('/tmp/oas.json', JSON.stringify(require('./
 
 ## Requirements
 
-- **Node.js 18+** (tested on 20 and 22)
+- **Node.js 20+** (matches `engines.node` in `package.json`; CI tests against 20 and 22)
 - **PostgreSQL 14+**
 - A modern Linux distribution (any currently supported LTS — Ubuntu 22.04 / 24.04, Debian 12, RHEL 9, etc.)