ax(mining): replace prose comment with usage example on respondWithMiningError

AX Principle 2 — comments show HOW with real values, not WHAT the function does.
"sends a structured error response" restates the signature; replaced with
two concrete call-site examples showing realistic arguments.

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

pkg/mining/service.go

@@ -100,8 +100,8 @@ func respondWithError(c *gin.Context, status int, code string, message string, d
 	c.JSON(status, apiErr)
 }
 
-// respondWithMiningError sends a structured error response from a MiningError.
-// This allows using pre-built error constructors from errors.go.
+// respondWithMiningError(c, ErrMinerNotFound("xmrig"))
+// respondWithMiningError(c, ErrInternal("failed to read config").WithCause(err))
 func respondWithMiningError(c *gin.Context, err *MiningError) {
 	details := ""
 	if err.Cause != nil {