Add .node-version file and use it in CI workflow

[ScottMorris]

Context

This repo hardcodes Node.js version 20 in the CI workflow:

- name: Setup Node.js
  uses: actions/setup-node@v4
  with:
    node-version: 20

Other repos in the org use a .node-version file referenced by CI workflows.

Proposal

1. Add a .node-version file (e.g., 20 or current LTS)
2. Update deploy-pages.yml to use node-version-file: '.node-version'

Acceptance Criteria

• .node-version file added
• CI workflow references the file instead of hardcoding
• Deploy passes