pr-babysit: overlap a fast review lane with the CI watch

Review findings are validated, fixed, and pushed without waiting on CI;
checkoutDir decouples the PR checkout from the session cwd. File-less CI
failures are scoped by a dedicated agent, paths canonicalized and
existence-checked via git ls-files, overlapping groups merged. Per-id
reply/resolve accounting retries failures and holds the green exit until
all outward work is drained.