ci: set up CI pipeline for automated testing and linting

[dimakis]

Context

Mitzo currently has no CI — tests and linting only run locally via pre-commit hooks. As the project grows and accumulates multi-layered bug fixes (see the user-message saga: PRs #47, #53, #54, #56, #57, #66, #79, #99, #100, #101), we need automated verification on every push and PR.

Scope

Phase 1: Core pipeline

• GitHub Actions workflow on push to main and all PRs
• npm test (vitest, 511+ tests)
• npm run lint (eslint)
• npm run format:check (prettier)
• TypeScript type-checking (tsc --noEmit)

Phase 2: Build verification

• npm run build — verify frontend builds cleanly
• Server compilation check
• Fail on unused exports or dead code (if tooling supports it)

Phase 3: Quality gates

• Branch protection on main — require CI pass before merge
• Test coverage reporting (optional, informational)
• Bundle size tracking (optional)

Notes

• Node version should match what's used in dev (check .nvmrc or package.json engines)
• SQLite (better-sqlite3) needs native compilation — may need build deps in the CI image
• Keep it fast — the test suite runs in ~1.5s locally, CI should stay under 2 minutes