Add CI check to enforce labels and milestone on PRs

Adds a GitHub Actions workflow that blocks PRs from merging unless they
have at least one label and a milestone assigned. Bot and draft PRs are
exempted. This replaces the manual process of tagging PRs.

Closes #1025

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rparolin

.github/workflows/pr-metadata-check.yml

@@ -0,0 +1,56 @@
+# SPDX-FileCopyrightText: Copyright (c) 2024-2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+name: "CI: Enforce label/milestone on PRs"
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+      - labeled
+      - unlabeled
+      - milestoned
+      - demilestoned
+
+jobs:
+  check-metadata:
+    name: PR has labels and milestone
+    if: github.repository_owner == 'nvidia'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check for labels and milestone
+        env:
+          LABELS: ${{ toJson(github.event.pull_request.labels) }}
+          MILESTONE: ${{ github.event.pull_request.milestone && github.event.pull_request.milestone.title || '' }}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          IS_BOT: ${{ github.actor == 'dependabot[bot]' || github.actor == 'pre-commit-ci[bot]' || github.actor == 'copy-pr-bot[bot]' }}
+          IS_DRAFT: ${{ github.event.pull_request.draft }}
+        run: |
+          if [ "$IS_BOT" = "true" ] || [ "$IS_DRAFT" = "true" ]; then
+            echo "Skipping check for bot or draft PR."
+            exit 0
+          fi
+
+          LABEL_COUNT=$(echo "$LABELS" | jq 'length')
+          ERRORS=""
+
+          if [ "$LABEL_COUNT" -eq 0 ]; then
+            ERRORS="${ERRORS}  - Missing labels: add at least one label to categorize this PR.\n"
+          fi
+
+          if [ -z "$MILESTONE" ]; then
+            ERRORS="${ERRORS}  - Missing milestone: assign a milestone to this PR.\n"
+          fi
+
+          if [ -n "$ERRORS" ]; then
+            echo "::error::This PR is missing required metadata:"
+            printf "$ERRORS"
+            echo ""
+            echo "Please update the PR at: $PR_URL"
+            exit 1
+          fi
+
+          echo "PR has $LABEL_COUNT label(s) and milestone '$MILESTONE'."