fix: fall back to git rev-parse in find_in_git_root

[kuishou68]

Summary

• wait briefly for the initial scan before reading the cached git root
• fall back to git rev-parse --show-toplevel when the cached git root is still unavailable
• add regression tests covering the startup-race fallback and the no-repo warning path

Notes

• I couldn't run the Neovim/Rust test stack in this sandbox because nvim and cargo are unavailable here.
• The patch is intentionally small and only changes find_in_git_root() plus focused Lua tests.

Closes #233

[dmtrKovalenko]

I have deprecated find in git root because the current find automatically does that right? Or not? I’m really unsure what your expected behavior here