codex auto-commit: the mail gate handles a repository with no commits yet #835

Found by CodeRabbit on #810 (pre-existing; the gate fails safe today by skipping). When the mail auto-commit repository has no commits yet (unborn HEAD), the baseline git rev-parse HEAD probe fails, so the gate skips the task's first commit — and every later task skips too while HEAD stays unborn.

What the fix does: the gate distinguishes an unborn HEAD (git symbolic-ref -q HEAD names a branch and git rev-parse --verify -q HEAD finds no commit) from a failed probe, at both the baseline and the final HEAD check. Unborn at baseline with a non-empty git status at the end commits as a new repository's first commit; unborn with an empty status is clean; any other probe failure stays an indeterminate skip naming the probe. Real-git tests: an unborn repo where the task writes files (committed), an unborn repo with no changes (clean), and a corrupted HEAD (indeterminate skip).

Refs #800, #810