feat: add Kiro commit attribution helper

[warren830]

Summary

• add an opt-in Kiro Stop hook recorder and Git prepare-commit-msg hook that append deterministic Kiro-Session-Id trailers
• add a one-command installer that respects core.hooksPath, preserves an existing hook, and handles managed unwritable hook directories
• register the current Kiro v1 hook schema with the permitted bash command form
• document coverage limitations so an absent trailer is not misinterpreted when the hook was never installed

This helper is independent of the Kiro S3 collection plugin series and can be reviewed or landed separately.

Validation

• sh -n on all three shell scripts
• shellcheck on all three shell scripts
• isolated install and --check smoke test in a temporary Git repository
• installer self-test verified both trailer creation after a recorded session and no inherited trailer without a new session

Signed-off-by: warren warren.chen830@gmail.com

[klesh]

LGTM