feat: add weekly GHCR cleanup workflow

[castrojo]

Summary

Adds a weekly scheduled workflow to prune dakota and dakota-nvidia image tags older than 90 days from GHCR.

Closes #664

Design

Mirrors projectbluefin/bluefin clean.yml exactly, using the shared org action instead of calling dataaxiom/ghcr-cleanup-action directly.

Setting	Value
Schedule	Sundays 00:15 UTC
Packages	dakota,dakota-nvidia
older-than	90 days
keep-n-tagged	7
keep-n-untagged	7
delete-orphaned-images	true

Dependency

Requires projectbluefin/actions#39 to merge first (bumps ghcr-cleanup action to v1.2.1 + adds delete-orphaned-images input). This PR is pinned to the feature branch SHA fb75840 for validation; will update to the @v1 tag once that PR merges.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 43dd65ba-067c-415e-91fc-a2e53db34f2f

📥 Commits

Reviewing files that changed from the base of the PR and between fc0803b and 792c59a.

📒 Files selected for processing (1)

• .github/workflows/clean.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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