Remove mcp section in agents.config.yaml

It's redundant. If a user wants to globally omit an MCP server, they can just remove it from mcp.config.

The ability to filter MCP server at a per agent level remains unchanged.

Start by locating agents.config.yaml and the code that loads or validates its mcp section, then check how mcp.config is handled. Remove the redundant global mcp section while preserving per-agent MCP filtering, and verify the runtime configuration behavior with the project's existing checks.

Written by the indexing model from the issue text.