Add agent skills for migration to mypyc and mypyc optimizations (#22072)

The idea is that these could be installed to a coding agent like Claude
Code or Codex, and the agent can use the instructions to migrate/prepare
code for mypyc compilation, and to optimize code for better performance
under mypyc. I have two separate skills, since it often makes sense to
first only do fairly minimal changes to get code to work correctly when
compiled, and further optimizations can be a later step. Also, to be
most effective, the optimization workflow needs some way to benchmark
and profile changes, whereas the migration workflow can just check that
code compiled and tests pass, so these are somewhat different use cases.

I used this with Claude Opus 5.5 to migrate a library at work to mypyc,
and the results looked reasonable (though I haven't deployed the change
the yet). This should be considered experimental still.

Author: JukkaL