feat(ci): staging counterpart of rebuild-on-dispatch.yml

[larsweiser]

Summary

Adds a staging-side mirror of rebuild-on-dispatch.yml. A single product-release-published dispatch from a product repo now refreshes both production and staging in parallel.

Closes the gap surfaced by the bare-SemVer integration test on 2026-05-13: staging was not refreshed by product-release dispatches, so verifying a new release on a draft-only product (e.g. pipeline-test-fixture) required a manual workflow_dispatch on deploy-staging.yml.

Changes

• .github/workflows/rebuild-on-dispatch-staging.yml (new, 58 lines)
  • Same trigger (repository_dispatch → product-release-published)
  • Checks out dev instead of main
  • PUBLIC_ENVIRONMENT: staging, command: deploy --env staging
  • Otherwise identical to the production workflow (Node 24, wrangler-action@v3, same env vars)

Convention match

Mirrors the per-environment-file pattern already in place across two other pairs:

Production	Staging
deploy-production.yml	deploy-staging.yml
rebuild-nightly.yml	rebuild-nightly-staging.yml
rebuild-on-dispatch.yml	rebuild-on-dispatch-staging.yml (this PR)

Routing note

repository_dispatch workflows only fire from files on the repo's default branch (main). So this file becomes active only after the next dev → main rollout. The header comment in the new file captures this for future readers.

Test plan

After this PR merges to dev and the subsequent dev → main rollout lands, cut a 0.0.3 release on pipeline-test-fixture (UI). Expected: the dispatch fires both workflows on this repo in parallel, both complete green, and staging picks up 0.0.3 without any manual workflow_dispatch.

• Branch CI green (no executable surface for this workflow on dev push — file only fires on dispatch from default branch)
• Merge to dev
• Prep dev → main rollout PR
• Merge rollout to main
• Cut 0.0.3 on pipeline-test-fixture via GitHub UI (CHANGELOG section + release)
• Verify both workflow runs appear in this repo's Actions tab: rebuild-on-dispatch (production) AND rebuild-on-dispatch-staging (this PR), both green
• Verify dev.blackbrowedlabs.com/produkte/pipeline-test-fixture/ version-chip shows 0.0.3 without a manual staging deploy
• Verify production side unchanged (/produkte/pipeline-test-fixture/ remains 404 — draft: true on the .md file)

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