ci(actions-caches): add daily cleaner for closed-PR pre-commit caches

[xmfcx]

• Adds .github/workflows/cleanup-actions-caches.yaml, a tiny daily workflow that lists pre-commit* Actions caches and deletes any whose attached PR is no longer open.
• Runs on schedule: 30 5 * * * (05:30 UTC) and on workflow_dispatch. Logs each cache decision and writes a markdown summary to $GITHUB_STEP_SUMMARY.

Why

GitHub's default cache LRU only evicts after 7 days of no access, so caches keyed to closed/merged PRs sit around uselessly. Today they are small (~430 MiB across 10 entries), but the cost is zero to keep them tidy and it scales as PR throughput grows.

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Test plan

• Manually dispatch the workflow from the Actions tab and confirm it completes green.

• Inspect the run's Summary for the 🧹 Pre-commit cache cleanup table (scanned / deleted / remaining counts).

• Expand the 🗂️ Inspecting per-PR state group and confirm OPEN PRs are marked ⏭️ keep and CLOSED/MERGED ones 🗑️ delete.

• After the run, verify deletions locally:

  gh cache list --repo autowarefoundation/autoware --limit 100 --key pre-commit \
    --json ref,key,sizeInBytes,createdAt

  Confirm no entries remain whose ref matches a closed PR.

[github-actions]

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[xmfcx]

Cute 🎀