Add MCP integration with MCPServer for tool-based interactions

- Introduce `MCPServer` to support connecting to MCP servers via stdio, SSE, or HTTP.
- Implement tool fetching, management, and invocation through MCP.
- Add `WithMCPServer` method to `ToolBox` for seamless tool integration.
- Extend schema package to handle raw JSON schemas for MCP tools.
- Update documentation with MCP usage guidelines and examples.

CLAUDE.md

@@ -20,6 +20,7 @@
   - `llm.go`: Contains core interfaces (`LLM`, `ChatCompletion`) and shared types (`Message`, `Role`, `Image`).
   - Provider implementations are in `openai.go`, `anthropic.go`, and `google.go`.
   - Schema definitions for tool calling are in the `schema/` directory.
+  - `mcp.go`: MCP (Model Context Protocol) client integration for connecting to MCP servers.
 - **Imports**: Organize imports into groups: standard library, then third-party libraries.
 - **Documentation**: Use standard Go doc comments for exported symbols.
 - **README.md**: The README.md file should always be kept up to date with any significant changes to the project.
@@ -45,3 +46,43 @@
 - `Ctrl+S` - Settings
 - `Ctrl+N` - New conversation
 - `Esc` - Exit/Cancel
+
+## MCP (Model Context Protocol) Support
+
+The library supports connecting to MCP servers to use their tools. MCP servers can be connected via:
+- **stdio**: Run a command as a subprocess
+- **sse**: Connect to an SSE endpoint
+- **http**: Connect to a streamable HTTP endpoint
+
+### Usage Example
+```go
+ctx := context.Background()
+
+// Create and connect to an MCP server
+server := &llm.MCPServer{
+    Name:    "my-server",
+    Command: "my-mcp-server",
+    Args:    []string{"--some-flag"},
+}
+if err := server.Connect(ctx); err != nil {
+    log.Fatal(err)
+}
+defer server.Close()
+
+// Add the server to a toolbox
+toolbox := llm.NewToolBox().WithMCPServer(server)
+
+// Use the toolbox in requests - MCP tools are automatically available
+req := llm.Request{
+    Messages: []llm.Message{{Role: llm.RoleUser, Text: "Use the MCP tool"}},
+    Toolbox:  toolbox,
+}
+```
+
+### MCPServer Options
+- `Name`: Friendly name for logging
+- `Command`: Command to run (for stdio transport)
+- `Args`: Command arguments
+- `Env`: Additional environment variables
+- `URL`: Endpoint URL (for sse/http transport)
+- `Transport`: "stdio" (default), "sse", or "http"