Controller code using the koa framework
Interesting question,

I understand you search for a rails-style way on how to write nodejs programs. However from a CodeReview perspective, your code should be idiomatic (conforming to the mode of expression characteristic of a language) so that other developers can grok your code and even maintain it.

From that perspective your code is quite good, the only thing being that you really (really) should put those semi-colons. Just get over it, good JavaScript has semicolons at the end of each line.

Other than that:


I like the use of 'use strict', generator functions and yield
I like the amount of comments, not so much the style, I would go for less vertical real estate

/*
 * User controller.
 */


could just as well be

//User controller

It's a matter of taste but I find this more idiomatic 

var skip = Math.max(0, this.query.skip), 
    limit = Math.min(10, this.query.limit);


than

var skip, limit
skip = Math.max(0, this.query.skip)
limit = Math.min(10, this.query.limit)


I guess from a style perspective you want to split declaration from initialization, but I find this takes too much vertical real estate.
Logging/Handling errors, you should consider putting more effort in handling errors than simply throwing a 403 on the client side, you should do something on the server side as well
Magic constants (404, 200) etc. are known well enough that you don't have to create named constants.


All in all I like your code, I guess if I had to maintain I would run it thru a script to fix all the semicolons and comments and go from there ;)