Extract build into reusable workflow to eliminate redundant clone+install+build across workflows

[Copilot]

deploy.yml and preview.yml each independently checked out, installed deps, and built — duplicating work on every PR and main push.

Changes

New: .github/workflows/build.yml

Reusable workflow_call workflow encapsulating the full build pipeline. Uploads dist/ as a short-lived artifact consumed by downstream deploy jobs. Accepts an optional base-url input for preview sub-path builds.

deploy.yml / preview.yml

• Replaced inline build steps with a build job that calls build.yml
• Added downstream deploy job that needs: build, downloads the artifact, and deploys
• preview.yml skips the build job on PR close (if: github.event.action != 'closed') while the deploy-preview cleanup job still runs via if: always()

Permissions hardening

Added explicit permissions blocks per job: contents: read for build, contents: write for gh-pages pushes, pull-requests: write for PR preview comments, permissions: {} on workflow_call caller jobs.

Original prompt

cache the build stage of the workflow so it can be reused across workflows instead of cloning and building redundantly

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