1) You know how to manage programmers, break up tasks, and do technical interviews. Good for you. Hire a freelancer, or several freelancers, and you'll get the job done.
2) You don't. You will be ripped off and dumped whoever you hire: freelancer or an agency. Just get a partner who is (1) and can manage people. Don't pay him any cash, or he will become effectively an agency that rips you off. Let him work for stock. If you can't, your idea is bad.
Simple proof: no VC or angel will invest into a startup who plans to implement his stuff by using an outsourced agency. They know it can't possibly work, and the very attempt means that would-be founders have no idea of what they are doing.
Why do i run an agency? Because being an individual freelancer has its limits. And i consider building my own project as a systematic mistake, too (or at least i never seen any successful projects started from scratch in my life, and i've been doing offshore development for 18 years).
I think of myself as of entertainment industry worker, rather as a producer of capital goods which i am supposed to be. My customers are not startup founders they think they are, they just mimick them. Everything they intend to build is so hopeless from the beginning it can't be seen otherwise: those who aren't hopeless, never hire agencies, and very rarely hire freelancers on the open market as well (knowing many of them are hidden agencies).
Most startup founders are nontechnical and finding engineers is still hard. Lots of businesses don’t even need engineers on staff all the time.
Why would a partner join you for $0 and stock? They know you can’t execute on your own, so why include you at all? If the tech partner would have been sold enough to work for free, they should do it without you. If you are bringing money, you can afford to pay them.
If that competence is not technical, fine: this is what technical cofounders are for. But if someone has an 'idea' of something without deep experience in a particular industry which makes him indispensable as a founder, that idea is no better than any of 5 or so 'ideas' i can invent if you buy me a couple beers in a bar. 'sitcom startup' as someone called these.
It also makes chances to get funding slim, even if idea is by some miracle a good one. Because there is no answer to 'what prevents a random guy from copying it' question, which every good investor must ask.