ci: add scheduled cache-warm workflow for BST remote cache

[castrojo]

Summary

Automation audit Phase 7 / ND1: scheduled workflow to keep the BST remote cache hot.

Problem: Dakota BST builds have a 420-min timeout. Full GNOME stack rebuild from scratch after junction ref bumps can exceed 6h.

Solution: Schedule Mon/Thu 06:00 UTC cache-warm build to keep artifacts fresh before typical work-hour PR windows.

Key choices:

• cancel-in-progress: false — let cache warm complete even if newer run starts
• || true on build — warming is best-effort, failure doesn't block anything
• Checks out testing branch — builds live content, not stale main
• oci/bluefin.bst — correct top-level build target per Justfile
• Includes Restore + Save cache steps for GitHub Actions cache layer

Supersedes #785 (clean branch from main, no conflicts).

Assisted-by: Claude Sonnet 4.5 via pi

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: b7a8d866-7c79-4656-8afe-43cb314d5d0e

📥 Commits

Reviewing files that changed from the base of the PR and between 5377a93 and a9afaad.

📒 Files selected for processing (1)

• .github/workflows/cache-warm.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/phase7-cache-warm-v2

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[github-actions]

Thanks for the PR! A maintainer will review it.

While you wait, make sure these pass locally:

just validate        # element graph check
just build default   # build the image
just boot-test       # confirm the desktop boots (exits 0 = pass)
just lint            # bootc container lint

If this PR fixes a bug, add verify steps to the linked issue so users can confirm the fix on their hardware after the next nightly ships:

```verify
ujust <something>   # what users should run to confirm the fix
```