[CI 03] ci: build and test workflow for backend and frontend

[amal66]

Summary

A CI workflow (.github/workflows/ci.yml) that builds and tests both packages and runs the eval harness on every PR and push to main. Ported from the reference index (#205, row amal66#46) and extended for this tree. Complements the e2e workflow in #220 — this one is the fast gate (~2-3 min), that one the full-stack gate.

Changes

Three jobs:

• backend — npm ci, npm test --if-present, npm run build (tsc, so the backend is typechecked on every PR).
• frontend — npm ci, npm test --if-present, npm run lint (advisory — continue-on-error, because main currently carries 23 eslint errors; flip it to blocking once that backlog is burned down), and a production next build with placeholder NEXT_PUBLIC_* env (values are only inlined at build time; nothing is contacted — verified locally that the build succeeds with placeholders).
• evals — runs node evals/run.mjs --threshold 1.0 when evals/ exists, else skips with a message.

Why

The repo has no CI besides the e2e workflow proposed in #220 today: a PR that breaks the backend build, fails tsc, or breaks a unit test reports green. Test steps use --if-present and the evals job checks for evals/run.mjs, so this is safe to merge before or after the other testing PRs in any order — on today's tree it still gates both builds.

Testing

• Every step verified locally against this branch's tree: backend npm ci && npm run build passes; frontend production build with the exact placeholder env in the workflow passes; actionlint-style read-through of the YAML (the workflow mirrors the fork's, which ran green on the fork's Actions).
• To make it merge-blocking, add the backend and frontend checks to branch protection alongside e2e / playwright (see docs/e2e-ci.md in [Testing 12] test: Playwright e2e suite + CI merge gate (auth, chat, projects, tabular, workflows) #220).

Based directly on main; no shared commits with the other testing PRs.

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