Enable auto-merge on planetscale-go sync PRs

[no-itsbackpack]

Summary

• After opening a planetscale-go sync PR, enable GitHub auto-merge with gh pr merge --auto --merge.
• Update the workflow/PR copy to reflect Approval Agent + auto-merge (tagging remains manual via the gorelease report).

Pairs with the APPROVAL_POLICY.md change in planetscale-go so sync PRs can land without a human +1 once Cursor approves and Buildkite is green.

Test plan

• Merge the planetscale-go APPROVAL_POLICY.md PR first (or alongside)
• Run sync-planetscale-go via workflow_dispatch (or next CLI release)
• Confirm the opened PR has auto-merge enabled
• Confirm it merges after Approval Agent +1 and required checks

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 868678e. Configure here.}

[no-itsbackpack]

Addressed Bugbot: Enable auto-merge now runs whenever the sync branch has changes, including re-runs where the branch/PR already exist.