Add MCP server management module

## Summary
Implemented a comprehensive MCP (Model Context Protocol) server management system as a Nix-Darwin module.

## Work Completed
- Created `nix-darwin/modules/mcp-servers.nix` with support for multiple installation types
- Support for binary, Docker, source code, and Git repository installations
- Smart caching mechanism to avoid unnecessary rebuilds
- Integration with 1Password for secret management via `op run`
- Automatic VS Code configuration generation
- All servers installed to `~/.mcp/servers/` with organized structure

## Technical Details
- Module uses content-based hashing for source builds
- Docker images can be exported as tar files for offline use
- Wrapper scripts handle environment setup and secret injection
- VS Code settings automatically updated via Home Manager activation

## Files Added
- `nix-darwin/modules/mcp-servers.nix` - Main module implementation
- `mcp/README.md` - Comprehensive documentation
- `mcp/examples/` - Example configurations for different server types
- `mcp/manage-servers.sh` - Server management utility
- `mcp/test-server.sh` - Testing utility

## Related
Part of the dotfiles repository enhancement effort.