feat(mcp): add Model Context Protocol integration

Implement comprehensive MCP support with stdio/HTTP/SSE transports, environment variable configuration (env and envFile), custom headers, tool registration, and automatic resource cleanup. Includes full test coverage and VSCode-compatible configuration.

- Added pkg/mcp/manager.go for server lifecycle management
- Added pkg/tools/mcp_tool.go for tool wrapping
- Integrated into agent loop with cleanup
- Support for envFile loading (.env format)
- Headers injection for HTTP/SSE authentication
- Example configs for filesystem, github, brave-search, postgres

pkg/tools/mcp_tool.go

@@ -0,0 +1,119 @@
+package tools
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+	mcpPkg "github.com/sipeed/picoclaw/pkg/mcp"
+)
+
+// MCPManager defines the interface for MCP manager operations
+// This allows for easier testing with mock implementations
+type MCPManager interface {
+	CallTool(ctx context.Context, serverName, toolName string, arguments map[string]interface{}) (*mcp.CallToolResult, error)
+}
+
+// MCPTool wraps an MCP tool to implement the Tool interface
+type MCPTool struct {
+	manager    MCPManager
+	serverName string
+	tool       *mcp.Tool
+}
+
+// NewMCPTool creates a new MCP tool wrapper
+func NewMCPTool(manager *mcpPkg.Manager, serverName string, tool *mcp.Tool) *MCPTool {
+	return &MCPTool{
+		manager:    manager,
+		serverName: serverName,
+		tool:       tool,
+	}
+}
+
+// Name returns the tool name, prefixed with the server name
+func (t *MCPTool) Name() string {
+	// Prefix with server name to avoid conflicts
+	return fmt.Sprintf("mcp_%s_%s", t.serverName, t.tool.Name)
+}
+
+// Description returns the tool description
+func (t *MCPTool) Description() string {
+	desc := t.tool.Description
+	if desc == "" {
+		desc = fmt.Sprintf("MCP tool from %s server", t.serverName)
+	}
+	// Add server info to description
+	return fmt.Sprintf("[MCP:%s] %s", t.serverName, desc)
+}
+
+// Parameters returns the tool parameters schema
+func (t *MCPTool) Parameters() map[string]interface{} {
+	// The InputSchema is already a JSON Schema object
+	schema := t.tool.InputSchema
+
+	// Convert to map[string]interface{} for compatibility
+	result := make(map[string]interface{})
+
+	// Use reflection to convert the schema
+	// The schema should already be in the correct format
+	if schema != nil {
+		// Attempt to convert directly
+		if schemaMap, ok := schema.(map[string]interface{}); ok {
+			return schemaMap
+		}
+
+		// Otherwise, build it manually
+		result["type"] = "object"
+		result["properties"] = map[string]interface{}{}
+		result["required"] = []string{}
+	} else {
+		// Default schema when nil
+		result["type"] = "object"
+		result["properties"] = map[string]interface{}{}
+		result["required"] = []string{}
+	}
+
+	return result
+}
+
+// Execute executes the MCP tool
+func (t *MCPTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
+	result, err := t.manager.CallTool(ctx, t.serverName, t.tool.Name, args)
+	if err != nil {
+		return ErrorResult(fmt.Sprintf("MCP tool execution failed: %v", err)).WithError(err)
+	}
+
+	// Handle error result from server
+	if result.IsError {
+		errMsg := extractContentText(result.Content)
+		return ErrorResult(fmt.Sprintf("MCP tool returned error: %s", errMsg)).
+			WithError(fmt.Errorf("MCP tool error: %s", errMsg))
+	}
+
+	// Extract text content from result
+	output := extractContentText(result.Content)
+
+	return &ToolResult{
+		ForLLM:  output,
+		IsError: false,
+	}
+}
+
+// extractContentText extracts text from MCP content array
+func extractContentText(content []mcp.Content) string {
+	var parts []string
+	for _, c := range content {
+		switch v := c.(type) {
+		case *mcp.TextContent:
+			parts = append(parts, v.Text)
+		case *mcp.ImageContent:
+			// For images, just indicate that an image was returned
+			parts = append(parts, fmt.Sprintf("[Image: %s]", v.MIMEType))
+		default:
+			// For other content types, use string representation
+			parts = append(parts, fmt.Sprintf("[Content: %T]", v))
+		}
+	}
+	return strings.Join(parts, "\n")
+}