fix: address review findings (escalation, marshal error, redundant check)

1. First-run escalation regression (MAJOR): Add post-posting escalation
   fallback. After posting APPROVED on first run, check if a sibling
   from the same user has REQUEST_CHANGES — if so, mark ours as
   superseded and re-post as REQUEST_CHANGES.

2. json.Marshal error handling (MINOR): Return error from EditComment
   instead of ignoring it with blank identifier.

3. Redundant condition (NIT): Remove dead assignment in reviewUnchanged
   where existingEvent was assigned from r.State then compared to itself.

gitea/client.go

@@ -407,7 +407,10 @@ func (c *Client) EditComment(ctx context.Context, owner, repo string, commentID
 	payload := struct {
 		Body string `json:"body"`
 	}{Body: newBody}
-	data, _ := json.Marshal(payload)
+	data, err := json.Marshal(payload)
+	if err != nil {
+		return fmt.Errorf("marshal edit payload: %w", err)
+	}
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodPatch, reqURL, bytes.NewReader(data))
 	if err != nil {