add github action to assign milestone automatically

.github/workflows/assign-milestone-on-close.yml

@@ -0,0 +1,40 @@
+name: Assign Milestone on Close
+
+env:
+  MILESTONE_ID: 80
+
+on:
+  issues:
+    types: [closed]
+  pull_request:
+    types: [closed]
+
+jobs:
+  assign-milestone:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check if milestone is set
+        id: check-milestone
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            const issueOrPr = context.payload.issue || context.payload.pull_request;
+            if (!issueOrPr.milestone) {
+              return {milestoneNotSet: true};
+            }
+            return {milestoneNotSet: false};
+
+      - name: Assign default milestone
+        if: steps.check-milestone.outputs.milestoneNotSet == 'true'
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            const issueOrPrNumber = (context.payload.issue || context.payload.pull_request).number;
+            const repository = context.repo;
+            await github.rest.issues.update({
+              ...repository,
+              issue_number: issueOrPrNumber,
+              milestone: process.env.MILESTONE_ID
+            });