Add workflow to auto-update open PRs when main changes

Triggers on every push to main, loops through all open PRs targeting
main, and merges the latest main into each branch via the GitHub API.
PRs with conflicts are skipped with a clear log message.

https://claude.ai/code/session_011MeXGRNe15ksjbwsDM8ULN

Author: claude

.github/workflows/update-prs.yml

@@ -0,0 +1,40 @@
+name: Update PRs with main
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  update-prs:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+
+    steps:
+      - name: Update all open PRs with latest main
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          pr_numbers=$(gh pr list \
+            --repo ${{ github.repository }} \
+            --base main \
+            --state open \
+            --json number \
+            --jq '.[].number')
+
+          if [ -z "$pr_numbers" ]; then
+            echo "No open PRs to update."
+            exit 0
+          fi
+
+          for pr in $pr_numbers; do
+            echo "Updating PR #$pr..."
+            if gh api repos/${{ github.repository }}/pulls/$pr/update-branch \
+              -X PUT \
+              -H "Accept: application/vnd.github+json" > /dev/null 2>&1; then
+              echo "  ✓ PR #$pr updated"
+            else
+              echo "  ✗ PR #$pr has conflicts or is already up to date — skipping"
+            fi
+          done