feat: add CI to detect references

.github/workflows/check-reference.yml

@@ -0,0 +1,39 @@
+name: Check patch references
+
+on:
+  push:
+    branches-ignore:
+      - dependabot/**
+  schedule:
+    # Once every day at midnight UTC
+    - cron: "0 0 * * *"
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout patches
+        uses: actions/checkout@v4
+
+      - name: Checkout core
+        uses: actions/checkout@v4
+        with:
+          repository: 'homebrew/homebrew-core'
+          path: homebrew-core
+
+      - name: Detect references
+        run: |
+          #!/bin/bash
+
+          core_patches=$(git -C homebrew-core grep -h "Homebrew/formula-patches" | awk -F/ '{print $NF}' | tr -d '"')
+          all_patches=$(git ls-files | grep -v "LICENSE" | grep -v "README.md" | grep -v ".github/workflows/triage-issues.yml" | grep -v ".github/workflows/check-reference.yml")
+          status=0
+
+          for patch in $all_patches; do
+            if ! grep -q "$core_patches" <<< "$patch"; then
+              echo "Unused patch: $patch."
+              # TODO: Delete the patch and commit the deletion.
+              status=1
+            fi
+          done
+          exit $status