Add PlanetScale branch metrics guidance

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Summary

• Update the PlanetScale skills to treat branch metrics as read-only evidence alongside Query Insights.
• Document the new metrics API endpoints and pscale metrics show, instant, and report commands for automation.
• Carry branch metrics through the orchestrator, inventory, operating model, best-practices matrix, report template, and CLI guidance.

Changelog source

• https://planetscale.com/changelog/metrics-api-and-cli

Why

PlanetScale now exposes Vitess and Postgres branch metrics through the public API and CLI, including time-series/current branch values, SQL query and tag-level metrics, Vitess tablet/table storage metrics, and Postgres resource metrics. The skills now guide agents to use those read-only surfaces for baselines and regression checks before drilling into Query Insights or other diagnostics.