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>