Add pytest CI to run tests on every PR

[linear-code]

Context

The repo has ~29 real unit tests but no CI workflow runs them on PRs. Current workflows only do black (format), docker build, publish, and the Claude review bots. A "smoke test CI" exists only as open PR #278. Without automated test runs, regressions can merge silently — and this backlog cleanup (#394) is riskier without it.

Task

• Merge/reimplement PR feat: add smoke test CI for PR regression testing #278 (smoke-test CI), or add a fresh pytest.yml workflow.
• Run pytest (+ ruff check) on every PR against main, Python 3.11–3.13.
• Make it a required status check before merge.

Definition of Done

• CI runs pytest + ruff on every PR
• Required check enabled on main
• Green on current main

Related: #394 (PR backlog triage). Source: repo review 2026-07.