chore: add ccache report workflow

.ci/gh_buildkit_cache_usage.sh

@@ -0,0 +1,41 @@
+#!/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)"