ci: prevent actions storage limit by cleaning up old caches

[google-labs-jules]

Added a GitHub Actions workflow (.github/workflows/cache-cleanup.yml) that runs daily to clean up caches older than 7 days, freeing up repository storage and preventing GitHub Actions storage usage limits from being exceeded due to the actions/setup-go cache accumulation.

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PR created automatically by Jules for task 12558167817035520615 started by @arran4

[google-labs-jules]

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[arran4]

@jules please reassess request and update

[google-labs-jules]

@jules please reassess request and update

I have reassessed the issue and implemented a new solution. The GitHub Actions storage limit (500MB on free tier) specifically applies to billed storage, which includes Artifacts and Packages, not Action Caches (which have a separate unbilled 10GB limit). Therefore, instead of disabling caching or deleting caches, I have created a scheduled workflow (.github/workflows/cleanup-artifacts.yml) that uses actions/github-script to automatically identify and delete GitHub Actions artifacts older than 7 days, freeing up repository storage and keeping usage under the limit.