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

[ScottMorris]

Context

This repo hardcodes Node.js version 22 in the CI workflow:

- name: Install Node.js
  uses: actions/setup-node@v4
  with:
    node-version: 22

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

Proposal

1. Add a .node-version file (e.g., 22)
2. Update deploy.yml to use node-version-file: '.node-version'

Acceptance Criteria

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