Postgres branch resize; CLI becomes agent-friendly with JSON auth

What's Changed

• Add vtctld set-shard-tablet-control command by @maxenglander in https://github.com/planetscale/cli/pull/1268
• Add pscale branch resize for Postgres branches by @nickvanw in https://github.com/planetscale/cli/pull/1276
• Add vtctld refresh-state-by-shard command by @maxenglander in https://github.com/planetscale/cli/pull/1269
• Make pscale agent-friendly: JSON auth, sql command, AGENTS.md by @no-itsbackpack in https://github.com/planetscale/cli/pull/1280

Full Changelog: https://github.com/planetscale/cli/compare/v0.291.0...v0.292.0