Support .github/poutine.yml as a config file discovery path

[graelo]

Hi, thanks again for poutine!

Currently, config auto-discovery only looks for .poutine.yml in the repo root. It would be nice to also support .github/poutine.yml as a discovery path (no leading dot when under .github/), similar to how other tools handle this:

• zizmor discovers .github/zizmor.yml
• GitHub's own config files (dependabot, CODEOWNERS, etc.) live under .github/

This would let users keep CI-related config files together under .github/ and reduce root-level clutter. The precedence could be: CLI --config flag > .poutine.yml (root) > .github/poutine.yml, so there's no backwards compatibility issue.

Cheers!