Add PlanetScale plugin

[SimeonGriggs]

Add PlanetScale plugin

Adds the PlanetScale plugin as an external entry in marketplace.json, sourced from planetscale/vscode-agent-plugin.

What it provides

• MCP Server: Authenticated access to PlanetScale organizations, databases, branches, schema, and query performance Insights via the hosted PlanetScale MCP endpoint (https://mcp.pscale.dev/mcp/planetscale). No local setup required.
• 4 Database Skills:
  • mysql — Schema design, indexing, query tuning, transactions, and operations for MySQL/InnoDB
  • postgres — Best practices, query optimization, MVCC, connection pooling, and PlanetScale-specific Postgres tooling
  • vitess — VSchema, sharding, vindexes, online DDL, and VReplication for PlanetScale Vitess databases
  • neki — Sharding readiness for PostgreSQL (PlanetScale's sharded Postgres)

Source repository

https://github.com/planetscale/vscode-agent-plugin

[patniko]

@copilot resolve the merge conflicts in this pull request