[codex] Pin project runtime to Node 22

[Dreamstore2046]

Summary

• add root engines.node policy for Node.js >=22.0.0
• add .nvmrc with 22 for local development
• update remaining GitHub Actions setup-node steps from Node 20 to Node 22
• document the Node 22 policy in CONTRIBUTING.md
• refresh package-lock.json so the root package metadata matches package.json

Closes #231

Validation

• npm install --package-lock-only --ignore-scripts --legacy-peer-deps
• node -e "const fs=require('fs'); const pkg=JSON.parse(fs.readFileSync('package.json','utf8')); const lock=JSON.parse(fs.readFileSync('package-lock.json','utf8')); if(pkg.engines.node!=='>=22.0.0') throw new Error('package engines mismatch'); if(lock.packages[''].engines.node!=='>=22.0.0') throw new Error('lock engines mismatch'); console.log('node engines pinned to '+pkg.engines.node);"
• rg "node-version:\\s*['\\\"]?20|node-version:\\s*20|node-version:\\s*\\\"20\\\"|node-version:\\s*'20'" .github/workflows (no matches)
• git diff --cached --check

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