ax(mining): replace APIError prose comment with usage example

AX principle 2 — comments must show HOW with real values, not restate
what the type declaration already says.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/service.go

@@ -49,7 +49,7 @@ type Service struct {
 	mcpServer           *ginmcp.GinMCP
 }
 
-// APIError represents a structured error response for the API
+// APIError{Code: "miner_not_found", Message: "xmrig not running", Retryable: false}
 type APIError struct {
 	Code       string `json:"code"`                 // Machine-readable error code
 	Message    string `json:"message"`              // Human-readable message