[MCP/Codeowners] Validate user

Ensure user has followed **Before you begin** section of docs here: https://eng.ms/docs/products/azure-developer-experience/onboard/access#request-access-to-azure-rest-api-and-sdk-repositories

🛠️ Tools:
* Given the current user validate that they meet all criteria of **Before you begin**. If not, show which parts are deficient.

💬 Prompts/instructions:
* Pre-requisites in [`copilot-instructions.md`](https://github.com/Azure/azure-rest-api-specs/blob/main/.github/copilot-instructions.md#sdk-generation-from-typespec) include the requirement for checking that a user can be a CODEOWNER
* Instructions for validating codeowner, instructions from **Before you begin**

❓Open questions
* How is output to SDK generation (other SDK agents) handled?

Start with .github/copilot-instructions.md at the SDK generation from TypeSpec section and review the linked Before you begin access requirements. Clarify how MCP/codeowners validation should identify deficient prerequisites and how output should be handled by other SDK agents; done means these behaviors and open questions are specified.

Written by the indexing model from the issue text.