feat: daily automated branch cleanup workflow

.github/workflows/branch-cleanup.yml

@@ -0,0 +1,67 @@
+name: Branch Cleanup
+
+on:
+  schedule:
+    - cron: '0 3 * * *'   # daily at 03:00 UTC
+  workflow_dispatch:        # allow manual trigger
+
+jobs:
+  delete-merged-branches:
+    name: Delete merged branches
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - name: Delete merged branches
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const protectedBranches = ['main', 'master', 'develop'];
+
+            // Resolve the repository's default branch dynamically
+            const { data: repo } = await github.rest.repos.get({
+              owner: context.repo.owner,
+              repo:  context.repo.repo,
+            });
+            const defaultBranch = repo.default_branch;
+            core.info(`Default branch: ${defaultBranch}`);
+
+            // Paginate through all branches
+            const branches = await github.paginate(github.rest.repos.listBranches, {
+              owner:    context.repo.owner,
+              repo:     context.repo.repo,
+              per_page: 100,
+            });
+
+            for (const branch of branches) {
+              if (protectedBranches.includes(branch.name)) {
+                core.info(`Skipping protected branch: ${branch.name}`);
+                continue;
+              }
+
+              // Check whether the branch has been fully merged into the default branch
+              let compare;
+              try {
+                ({ data: compare } = await github.rest.repos.compareCommitsWithBasehead({
+                  owner:    context.repo.owner,
+                  repo:     context.repo.repo,
+                  basehead: `${defaultBranch}...${branch.name}`,
+                }));
+              } catch (err) {
+                core.info(`Could not compare ${branch.name} – skipping (${err.message})`);
+                continue;
+              }
+
+              if (compare.ahead_by === 0) {
+                core.info(`Deleting merged branch: ${branch.name}`);
+                await github.rest.git.deleteRef({
+                  owner: context.repo.owner,
+                  repo:  context.repo.repo,
+                  ref:   `heads/${branch.name}`,
+                });
+              } else {
+                core.info(`Branch ${branch.name} has ${compare.ahead_by} unmerged commit(s) – skipping`);
+              }
+            }