CI: lint, typecheck, build workflow

[HerbHall]

Description

GitHub Actions workflow with parallel jobs for Go and frontend CI.

Deliverables

• .github/workflows/ci.yml
• Jobs: Go Lint + Build + Test, Frontend Lint + TypeCheck + Build, Docker Build

Acceptance Criteria

• CI runs on push to main and on PRs
• Go job: go build, go test, golangci-lint
• Frontend job: tsc --noEmit, eslint, npm run build
• Docker job: docker build succeeds

Wave

Wave 4 (independent).