CI: restore build step and Windows test matrix dropped from upstream #75

Follow-up to #6. Upstream ci.yml runs pnpm build and the test suite on ubuntu-latest and windows-latest. The fork's workflow (a6c5417) replaced it with typecheck / lint / test on Ubuntu only (plus claude plugin validate).

• Without a build step CI misses electron-vite and CLI bundle (vite.cli.config.ts) failures.
• Without Windows CI misses path handling bugs (project path encoding, pathDecoder, separators).
• pnpm test:coverage:critical (vitest.critical.config.ts) is not run in CI, and its coverage set (guards, configValidation, pathDecoder, ProjectPathResolver) contains none of the calculation code.

Что сделать

1. Добавить шаги pnpm build и pnpm build:cli в .github/workflows/ci.yml.
2. Матрица ubuntu-latest / windows-latest для тестов (fail-fast: false, как в апстриме), сохранив --frozen-lockfile.
3. Добавить в coverage.include critical-конфига src/main/utils/jsonl.ts, src/shared/turnAccounting.ts, src/cli/analyzeSession.ts, src/renderer/utils/contextTracker.ts и запускать pnpm test:coverage:critical в CI.

Acceptance

• Зелёный CI на Ubuntu и Windows
• Сборка приложения и CLI проверяется на каждом PR
• Порог critical coverage включает код расчётов и проверяется в CI