feat(marlowe): add gh credential helper to local gitconfig

Move the GitHub credential helper config (written by `gh auth
setup-git`) into the marlowe-specific .gitconfig_local so it
doesn't pollute the shared common gitconfig.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: FridrichMethod

marlowe/git/.gitconfig_local

@@ -0,0 +1,6 @@
+[credential "https://github.com"]
+	helper =
+	helper = !/users/zyli2002/micromamba/envs/login/bin/gh auth git-credential
+[credential "https://gist.github.com"]
+	helper =
+	helper = !/users/zyli2002/micromamba/envs/login/bin/gh auth git-credential