Add APPROVAL_POLICY for CLI sync auto-approval

[no-itsbackpack]

Summary

• Add root APPROVAL_POLICY.md so Cursor PR Routing & Approval can auto-approve mechanical planetscale-cli[bot] sync PRs.
• Low-severity Bugbot findings are explicitly not a hard stop; Medium/High still require Surfaces.

Test plan

• Confirm Approval Agent is scoped to planetscale/planetscale-go and reads APPROVAL_POLICY.md
• On the next sync PR (or re-trigger on Sync API client from cli@40cd697 #338), verify Approval Agent approves despite Low Bugbot findings
• Confirm non-sync PRs are still routed to Surfaces and not auto-approved

[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 68ecc6a. Configure here.}

[no-itsbackpack]

Addressed Bugbot: sync auto-approve now requires bot author and matching branch/title (not any).