chore: Add shared validate-pr composite action

Add the PR validation workflow using the shared composite action from
getsentry/github-workflows#153.

Validates non-maintainer PRs against contribution guidelines and enforces
draft status on all new PRs.

#skip-changelog

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

Author: stephanie-anderson

.github/workflows/validate-pr.yml

@@ -0,0 +1,16 @@
+name: Validate PR
+
+on:
+  pull_request_target:
+    types: [opened, reopened]
+
+jobs:
+  validate-pr:
+    runs-on: ubuntu-24.04
+    permissions:
+      pull-requests: write
+    steps:
+      - uses: getsentry/github-workflows/validate-pr@4243265ac9cc3ee5b89ad2b30c3797ac8483d63a
+        with:
+          app-id: ${{ vars.SDK_MAINTAINER_BOT_APP_ID }}
+          private-key: ${{ secrets.SDK_MAINTAINER_BOT_PRIVATE_KEY }}