chore: bump Node to 22 across CI and add engines pin

[sserrata]

Summary

• Bumps actions/setup-node version from '20' → '22' in both deploy-preview.yml (2 refs) and deploy-live.yml (2 refs)
• Adds .nvmrc pinning Node 22 for local dev / IDE tooling
• Adds "engines": { "node": ">=22.0.0" } to package.json so yarn install gives contributors a clear signal if they're on an older Node

Why

• Node 20 goes EOL 2026-04
• Node 22 is the current LTS
• Unblocks dev dependencies that already require Node 22 (e.g. Dependabot PR Bump lint-staged from 16.4.0 to 17.0.8 #1330 bumping lint-staged 16 → 17, which needs >=22.22.1)

Impact

• CI: all workflows now use Node 22
• Local dev: contributors on Node 20 will get an engine warning on their next yarn install; they'll need to bump (nvm use will pick up the new .nvmrc)
• Runtime: none — the site is static, Node is only used at build time
• Compatibility: Docusaurus 3.10 supports Node 22

Test plan

• CI build passes on Node 22
• Deploy preview succeeds
• After merge: comment @dependabot rebase on Bump lint-staged from 16.4.0 to 17.0.8 #1330 so lint-staged v17 installs cleanly

🤖 Generated with Claude Code

[sserrata]

Parking this until closer to Node 20 EOL (2026-04). The workflow bump is provably correct but can't self-validate due to pull_request_target reading workflows from master. Will revisit — likely as a coordinated PR that includes contributor communication, or via admin-merge closer to the deadline. Branch chore/node-22 deleted.