Migrate repository from Yarn to npm

[rzhao271]

This repository appears to use Yarn (yarn.lock present). Please migrate package management from Yarn to npm.

Scope:

• Replace Yarn commands/workflows with npm equivalents.
• Generate and commit package-lock.json.
• Remove yarn.lock and Yarn-only config files if they are no longer needed.
• Update CI/scripts/docs to use npm (for example, npm ci and npm run <script>).

Acceptance criteria:

• No remaining Yarn usage in scripts, docs, or CI pipelines.
• package-lock.json is committed and up to date.
• Build/test workflows pass after migration.