CI: run production build in GitHub Actions (not only local checks)

[saif-at-scalekit]

Context

PR #635 (preview/add-ci-workflow) adds CI checks, but it currently runs validation only (pnpm astro check, pnpm run format:check, markdown audit) and does not run the actual production build.

Goal

Ensure the same build that is expected locally is executed in GitHub Actions so build regressions are caught before merge.

Proposed change

Update .github/workflows/ci.yml (introduced in preview/add-ci-workflow) to include a production build step, e.g. pnpm run build (or the canonical repo build command).

Acceptance criteria

• CI workflow includes a build step after dependency installation/checks
• Pull requests fail when the build fails
• Build command in CI matches documented local build command

References

• PR: build: add CI workflow for format check and markdown audit #635
• Branch: preview/add-ci-workflow