git-story: register git pre-commit hook

[Dominic Nguyen (datduyng)]

Stacked on #3101.

git story init now also writes a pre-commit script to the git hooks directory. The script runs exec git story hooks git pre-commit "$@", so git's hook arguments and stdin reach the command unchanged. The handler reads stdin and prints a placeholder.

• Honors core.hooksPath and worktrees via git rev-parse --git-path.
• Does not overwrite a hook it did not write (marker line); exits 1 and reports it.

Verified in a temp repo: commit fires the hook; echo piped | .git/hooks/pre-commit a b prints args=["a","b"] stdin="piped\n"; re-run is idempotent; foreign hook is skipped.

[Dominic Nguyen (datduyng)]

Split into #3104 (hook + init) and the test PR stacked on it.