feat: add reusable cache-cleanup workflow

[mbologna]

Summary

• Adds a reusable cache-cleanup.yml workflow that deletes GitHub Actions cache entries older than a configurable threshold (default: 7 days)
• Prevents Docker buildx GHA cache from exceeding the 10 GB per-repo limit, which causes failed to reserve cache build failures
• Uses actions/github-script to paginate through and delete stale entries

Usage

Calling repos add a scheduled workflow:

on:
  schedule:
    - cron: "0 4 * * 0"

jobs:
  cache-cleanup:
    uses: mbologna/.github/.github/workflows/cache-cleanup.yml@main
    with:
      max_age_days: 7

Test plan

• Merge and reference from docker-bitlbee CI
• Verify cache entries are cleaned up after scheduled run

🤖 Generated with Claude Code