please take a look at http://wiki.sharpdevelop.net/JoiningTheTeam.ashx for the formal details of contributing to SharpDevelop.
LINQ to SQL: take a look at the ICSharpCode.Data.AddIn, it already implements this for Entity Framework. But it can certainly use some improvements.
Refactoring: We are currently rewriting the refactoring engine, take a look at https://github.com/icsharpcode/NRefactory/ for the new version. NRefactory 5 is not yet fully integrated into SharpDevelop, you can find the work-in-progress on the "newNR" branch in the SharpDevelop git repository. However, it's not yet ready to start building refactorings on top of it.