Add .github/workflows/autoupdate.yml. On every push to main, the chinthakagodawita/autoupdate action merges main into all open PRs that target it (PR_FILTER: "all"), keeping branches current as PRs merge. The action is pinned to commit 0707656 (v1.7.0) for supply-chain safety. Uses the default GITHUB_TOKEN with minimal contents:write and pull-requests:write permissions. Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>