ci: gate PRs on ruff, eslint and frontend build

[lfnovo]

Summary

• Adds three CI jobs to .github/workflows/test.yml: Backend Lint (ruff check .), Frontend Lint (npm run lint) and Frontend Build (npm run build)
• These commands are documented as required in AGENTS.md but were not enforced on PRs until now
• Jobs reuse the existing setup patterns (uv with cache, Node 22 with npm cache) and run in parallel with the test jobs
• mypy is intentionally out of scope (separate PR later)

All three commands pass locally on current main.

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

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