chore: add ccache report workflow

.ci/gh_buildkit_cache_usage.sh

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+#!/bin/bash
+# Reports total count and size of GitHub Actions caches whose key starts with a given prefix.
+# Used for BuildKit GHA cache (keys like buildkit-blob-1-sha256:...).
+#
+# Requires: gh (GitHub CLI), jq
+# Run from repo root with gh auth login already done (or GITHUB_TOKEN set in CI).
+#
+# Usage: .ci/gh_buildkit_cache_usage.sh [key-prefix]
+#   Default key-prefix: buildkit-blob
+#
+# Example: .ci/gh_buildkit_cache_usage.sh
+# Example: .ci/gh_buildkit_cache_usage.sh buildkit-blob
+
+set -eo pipefail
+
+KEY_PREFIX="${1:-buildkit-blob}"
+LIMIT=1000
+
+if ! command -v gh &>/dev/null; then
+  echo "Error: gh (GitHub CLI) is required. Install from https://cli.github.com/" >&2
+  exit 1
+fi
+if ! command -v jq &>/dev/null; then
+  echo "Error: jq is required. Install with: brew install jq" >&2
+  exit 1
+fi
+
+json=$(gh cache list --key "$KEY_PREFIX" --limit "$LIMIT" --json key,sizeInBytes 2>/dev/null) || {
+  echo "Error: gh cache list failed (check gh auth or repo access)" >&2
+  exit 1
+}
+
+count=$(echo "$json" | jq 'length')
+total_bytes=$(echo "$json" | jq 'map(.sizeInBytes) | add // 0')
+
+# Human-readable size (GiB, 2 decimals)
+total_gib=$(echo "$total_bytes" | jq -r '. / 1024 / 1024 / 1024 | . * 100 | floor / 100')
+
+echo "Cache key prefix: $KEY_PREFIX"
+echo "Entries: $count"
+echo "Total size: $total_bytes bytes ($total_gib GiB)"

.github/workflows/cache-report.yml

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+# Reports total GitHub Actions cache usage for BuildKit blob caches (used by docker-release).
+# Run locally: .ci/gh_buildkit_cache_usage.sh
+
+name: Cache usage report
+
+on:
+  schedule:
+    # Mondays 09:00 UTC
+    - cron: '0 9 * * 1'
+  workflow_dispatch:
+
+jobs:
+  report:
+    name: buildkit-blob cache usage
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Install jq
+        run: sudo apt-get update && sudo apt-get install -y jq
+
+      - name: Report cache usage
+        id: cache_report
+        run: |
+          ./.ci/gh_buildkit_cache_usage.sh buildkit-blob | tee report.txt
+          echo "summary<<EOF" >> "$GITHUB_STEP_SUMMARY"
+          cat report.txt >> "$GITHUB_STEP_SUMMARY"
+          echo "EOF" >> "$GITHUB_STEP_SUMMARY"