Add a skill for JIT optimization workflow

In order to minimize the developer loop for Copilot when it's tasked to implement an optimization we should prepare an instruction (skill, to avoid polluting the general instructions).

I'm thinking of a "take the function from an issue, create a console app where you invoke it (might need to simulate Tier1 promotion) and record SPMI context for that function only. Then, work with the context only. Once finished, run SPMI replay to validate it on a broader set. Then normal tests.

cc @dotnet/jit-contrib