ci: unify Node.js version to 22 (drop EOL 18 and outdated 20)

[lml2468]

Problem

The CI matrix tests against Node.js [18, 20]:

• Node 18 reached EOL in April 2025
• Node 20 enters maintenance mode in October 2026
• octo-adapters already uses Node 22

Testing against two outdated versions wastes CI minutes and is misaligned with the org standard.

Proposed change

Replace the matrix with a single Node.js 22 (current LTS) target in .github/workflows/ci.yml.

Acceptance

• .github/workflows/ci.yml uses node-version: 22 directly
• YAML syntax verified (actionlint)
• CI passes on Node 22

Implemented by #73.

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