ci: add build and audit gate

[MiguelMedeiros]

Summary

• Adds a GitHub Actions CI gate for the Discord client.
• Runs npm ci, npm run build, and npm audit --audit-level=high on PRs and main pushes.

Test plan

• Ran the same commands in Docker/Node 20 on zero.
• npm ci OK
• npm run build OK
• npm audit --audit-level=high OK, zero vulnerabilities

Release: Murray Rothbot v2.0.0 maintenance baseline.