Lazy-load MCP servers on first tool invocation

### Describe the feature or problem you'd like to solve

Currently, all MCP servers configured in mcp-config.json are connected at CLI startup. As users add more MCP servers (ADO, GitHub, Work IQ, custom agents, etc.), this increases startup time even when most servers won't be used in a given session.

### Proposed solution

Add a lazy-loading option so MCP servers connect on-demand — only when a tool from that server is first invoked. This could be a per-server config flag like "lazy": true, or a global setting.

Benefits:

- Faster startup time, especially for users with many configured servers
- Reduced resource usage for servers that aren't needed in every session
- Better scalability as the MCP ecosystem grows

### Example prompts or workflows

_No response_

### Additional context

_No response_

Empieza rastreando cómo se lee mcp-config.json y cómo se conectan todos los servidores MCP durante el inicio de la CLI. Después, sigue el recorrido desde la primera invocación de una herramienta hasta el servidor seleccionado. Se considera terminado cuando una configuración de carga diferida por servidor o global conecta únicamente el servidor necesario en el primer uso, preservando el comportamiento configurado del servidor.

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