feat(ci): add greenhouse PR preview workflow

[ArtieReus]

Summary

Add a new GitHub Actions workflow to build Docker images for Greenhouse PR previews. The workflow builds images when the greenhouse-pr-build label is present, manages the greenhouse-pr-preview label for ArgoCD deployment, and cleans up images
when PRs are closed.

Changes Made

• Add .github/workflows/build-push-greenhouse-pr-preview.yaml workflow
• Workflow triggers on PR label, synchronize, opened, reopened, and closed events
• Build job creates Docker image with tag pr-{number}-{sha} when greenhouse-pr-build label is present
• On new commits, removes preview label before rebuild and re-adds after success (triggers ArgoCD redeployment)
• Cleanup job deletes all PR images using crane when PR is closed/merged
• Uses separate image name juno-app-greenhouse-pr-preview to avoid collision with production images

Related Issues

Testing Instructions

1. Create a PR with changes to apps/greenhouse/**
2. Add the greenhouse-pr-build label to the PR
3. Verify the workflow runs and builds the image
4. Check that greenhouse-pr-preview label is added after successful build
5. Push a new commit to the PR
6. Verify the preview label is removed, image rebuilt, and label re-added
7. Close the PR and verify images are cleaned up

Checklist

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have made corresponding changes to the documentation (if applicable).
• My changes generate no new warnings or errors.
• I have created a changeset for my changes.

PR Manifesto

Review the PR Manifesto for best practises.

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Latest commit: 92cf0f6

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[Copilot]

Pull request overview

Adds a dedicated GitHub Actions workflow to build/push Docker images for Greenhouse PR preview deployments, controlled via PR labels, and to clean up preview images when PRs close.

Changes:

• Introduces a PR-triggered workflow that builds and pushes a pr-{number}-{sha}-tagged image when greenhouse-pr-build is present.
• Manages the greenhouse-pr-preview label to trigger ArgoCD redeployments on new commits.
• Adds a cleanup job to delete PR-scoped image tags on PR close.

[Copilot]

Pull request overview

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Pull request overview

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Pull request overview

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