[tuhaihe]

Merging a PR by hand often means waiting for the last CI job to finish after it has already been approved. Turn on GitHub's auto-merge so a committer can arm a PR once its commit history is ready, and GitHub merges it after the required reviews and status checks pass. PRs that still need squashing or other history cleanup should keep being merged by hand.

Also document the existing allow_update_branch and del_branch_on_merge settings under pull_requests.

This follows the same change in the main repo, apache/cloudberry commit 977a348.

See: https://lists.apache.org/thread/ys6k2g1jvoookhw788vhrrjpr24z31sp

Assisted-by: Claude Code

closes: #ISSUE_Number

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