Port git extension scripts to Python

[mnriem]

Part of #3277. Depends on the check-prerequisites PoC.

Tasks

• Port extensions/git/scripts/{bash,powershell}/ — git-common, initialize-repo, auto-commit, create-new-feature-branch — to Python.

Acceptance

• Parity tests on branch creation and commit behavior.

[marcelsafin]

I'd like to take this one too, same pattern as #3386 and #3387: standalone Python ports under extensions/git/scripts/python/, no runtime wiring changes, parity tests that run bash and Python side by side on branch creation and commit behavior.