feat(cli): background watch daemon + cursor-based read for the react-grab skill

`grab watch` now starts a detached, idempotent-per-dir daemon that captures
grabs to history.jsonl, and `grab read` consumes them via a cursor
(exactly-once, bounded output, optional --wait). This lets the skill run
hands-free instead of relying on a blocking command the agent harness cancels.
Adds daemon lifecycle + grab-log utils, unit tests, and a rewritten skill.