Feature Request: Support for JSON string input and dynamic MCP server configuration

Use Case:
• Allow passing MCP configuration as a JSON string via command line
• Enable integration with password managers (like 1Password CLI)
• Support dynamic selection of MCP servers at startup

Benefits:
1. Improved security: Allows fetching secrets at runtime from tools like 1Password using op item get ... or aws ssm get-parameter ... rather than storing them in config files
2. Modular configuration: Allows keeping individual MCP server configs in a mcp-server.d/ directory for better organization
3. Better workflow: Allows using tools like fzf --multi to interactively select which MCP servers to load at startup
4. Scripting flexibility: Enable shell scripts to dynamically generate and pass MCP configurations

Implementation Suggestion:

Add a command line flag (e.g., mcp json) that accepts a JSON string containing MCP server configurations, which would take precedence over file-based configurations.