Add contributing guidelines to prevent seed.json PRs

[rdimitrov]

Motivation and Context

We've been receiving numerous PRs from authors attempting to publish their MCP server by adding entries to data/seed.json. This is not the correct approach as servers should be published using the mcp-publisher CLI tool.

Many of these PRs appear to be AI-generated so the existing docs aren't being surfaced effectively. This PR adds explicit guidance in the files that AI coding assistants actually read:

• CONTRIBUTING.md - shown by GitHub when opening new PRs; widely ingested by AI tools
• .github/copilot-instructions.md - read by GitHub Copilot
• CLAUDE.md - read by Claude Code and Claude on github.com

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context