feat: skip re-posting when review is unchanged (preserve threads)

Before posting, compare the new review body+event against the existing
review with the same sentinel. If identical, skip entirely — this
preserves conversation threads on inline comments and avoids
re-notifying reviewers for findings they already know about.

Only re-posts when findings actually change (fixed, new, or different).

Tests: 6 cases covering identical, different body, different state,
stale reviews, and different sentinels.

cmd/review-bot/main.go

@@ -257,6 +257,15 @@ func main() {
 		log.Printf("Attaching %d inline comments", len(inlineComments))
 	}
 
+	// Check if existing review is unchanged — skip to preserve conversation threads
+	if *updateExisting && *reviewerName != "" {
+		existingReviews, err := giteaClient.ListReviews(ctx, owner, repoName, prNumber)
+		if err == nil && reviewUnchanged(existingReviews, reviewBody, event, sentinel) {
+			log.Printf("Review unchanged from previous run; skipping to preserve threads")
+			return
+		}
+	}
+
 	log.Printf("Posting review (event=%s)...", event)
 	posted, err := giteaClient.PostReview(ctx, owner, repoName, prNumber, event, reviewBody, inlineComments)
 	if err != nil {
@@ -465,3 +474,23 @@ func shouldEscalate(reviews []gitea.Review, postedID int64, postedLogin, ownSent
 	}
 	return false
 }
+
+// reviewUnchanged checks if an existing review with the same sentinel
+// already has identical body and state. Returns true if a re-post would
+// produce the same result (skip to preserve conversation threads).
+func reviewUnchanged(reviews []gitea.Review, newBody, newEvent, sentinel string) bool {
+	for _, r := range reviews {
+		if r.Stale {
+			continue
+		}
+		if !strings.Contains(r.Body, sentinel) {
+			continue
+		}
+		// Compare state (map APPROVED back from Gitea's representation)
+		existingEvent := r.State
+		if existingEvent == r.State && existingEvent == newEvent && r.Body == newBody {
+			return true
+		}
+	}
+	return false
+}