fix: skip posting review when HEAD moves during evaluation

When a new push arrives while review-bot is processing, the review
would be posted against a stale commit. This causes noise in the
PR timeline with findings that reference code that no longer exists.

Before posting, re-fetch PR metadata and compare HEAD SHA with the
commit we evaluated against. If they differ, log a warning and exit
successfully — a new workflow run should already be processing the
new HEAD.

Fixes #52

cmd/review-bot/main.go

@@ -315,6 +315,23 @@ func main() {
 
 	sentinel := fmt.Sprintf("<!-- review-bot:%s -->", *reviewerName)
 
+	// Stale check: verify HEAD hasn't moved since we started
+	// Re-fetch PR metadata to get the current HEAD SHA
+	evaluatedSHA := pr.Head.Sha
+	currentPR, err := giteaClient.GetPullRequest(ctx, owner, repoName, prNumber)
+	if err != nil {
+		slog.Warn("could not re-fetch PR for stale check", "pr", prNumber, "error", err)
+		// Continue anyway — better to post a potentially stale review than fail
+	} else if currentPR.Head.Sha != evaluatedSHA {
+		slog.Warn("HEAD moved during review — skipping stale review",
+			"evaluated", evaluatedSHA,
+			"current", currentPR.Head.Sha,
+			"pr", prNumber)
+		// Exit successfully — this isn't an error, just outdated work
+		// A new workflow run should already be in progress for the new HEAD
+		return
+	}
+
 	// Map findings to inline comments for lines present in the diff
 	diffRanges := gitea.ParseDiffNewLines(diff)
 	var inlineComments []gitea.ReviewComment