git-story: add pre-commit hook end-to-end test

[Dominic Nguyen (datduyng)]

Stacked on #3104.

Adds jest to git-story (same setup as aiclient) and one end-to-end test:

1. git story init in a temp repo.
2. A real git commit runs the hook through git's sh.
3. Runs the hook with an argument and piped stdin, and checks both reach the handler.

The test uses an empty global git config and a git-story shim on PATH. Paths use path.delimiter and forward slashes, so the test runs on Windows, macOS, and Linux.