ci: add a test task to turbo + CI pipeline

[Mona-i]

Description

turbo.json (L3–L17) defines build, dev, typecheck, lint, and clean, but there is no test task, and no app defines a test script — so there is no test pipeline for the whole repo. (This is the umbrella/config counterpart to standing up the API tests; the two should land together or in sequence.)

Scope

• Add a test task to turbo.json with appropriate dependsOn/caching.
• Add a root test script (turbo run test) to package.json.
• Add a Test step to .github/workflows/ci.yml after build.
• Ensure it's green even before every app has tests (a no-test package should not fail the pipeline).

Out of scope

• Writing the actual app tests (tracked separately) — this issue is the plumbing.

Acceptance criteria

• pnpm test runs turbo run test across the workspace without error.
• CI has a test step.
• Packages without tests don't break the run.

Where to start

• turbo.json, root package.json, .github/workflows/ci.yml.

Estimated effort: ~2–3h. Good first issue.