Update PlanetScale skills for new Vitess CLI operations

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Summary

Updates PlanetScale skill guidance for new Vitess CLI operations announced in the changelog:

• https://planetscale.com/changelog/cli-maintenance-schedules
• https://planetscale.com/changelog/cli-database-throttler
• https://planetscale.com/changelog/cli-deploy-request-force-cutover

What changed

• Added read-only inventory guidance for Vitess maintenance schedules and database-level migration throttler state.
• Updated Vitess deploy-request safety guidance for database-level throttler defaults and the new CLI force-cutover command.
• Classified throttler mutations and production force cutover in the change-gates approval model.
• Reflected the new operational surfaces in the best-practices matrix, agent operating model, and CLI automation skill.

Why

These changelog entries add CLI surfaces that materially affect agent guidance: agents can now inspect maintenance schedules and throttler defaults safely, while throttler updates and force cutover can change deploy behavior or production availability and need explicit approval gates.

Verification

• git diff --check HEAD~1..HEAD