Add commitID parameter to gitea.Client.PostReview so the review is
anchored to the specific commit that was evaluated. The caller
(cmd/review-bot) already computes evaluatedSHA from pr.Head.Sha;
this wires it through to the Gitea API payload.
When commitID is empty, omitempty drops it from the JSON and Gitea
defaults to the current PR head (backward-compatible).
Closes#107