git-story: register git pre-commit hook

[Dominic Nguyen (datduyng)]

#!/bin/sh
# git-story hook
exec git-story hooks git pre-commit "$@"

exec passes git's hook arguments and stdin to git story hooks git pre-commit unchanged. The handler reads stdin and prints a placeholder. Git runs hooks with sh on all platforms, including Git for Windows.

• Uses git rev-parse --git-path, so core.hooksPath and worktrees work.
• Does not overwrite a hook it did not write (marker line). It reports the hook and exits 1.