Test bullet parser; fix marker-less passthrough

The previous parser treated any non-empty non-header line as a bullet,
so 'plain paragraph' answers were silently rewrapped as <ul><li>. Now
we only count a line if it had a recognized bullet marker (- * • ·);
if none did, fall back to raw + <pre> as originally intended.

Tests cover dash/asterisk/unicode bullets, blank-line tolerance,
Summary:/TL;DR header stripping, HTML escaping, single-bullet output,
the no-marker passthrough path, and the IsQuestionReaction set.

internal/explainer/explainer.go

@@ -139,29 +139,36 @@ func (e *Explainer) summarize(headline, body string) (string, error) {
 func formatSummary(raw string) (plain, htmlBody string) {
 	lines := strings.Split(raw, "\n")
 	var bullets []string
+	sawMarker := false
 	for _, line := range lines {
 		line = strings.TrimSpace(line)
 		if line == "" {
 			continue
 		}
-		// Normalize common bullet markers
-		for _, prefix := range []string{"- ", "* ", "• ", "·"} {
-			if strings.HasPrefix(line, prefix) {
-				line = strings.TrimSpace(line[len(prefix):])
-				break
-			}
-		}
 		// Skip header-ish lines the model sometimes adds
 		low := strings.ToLower(line)
 		if strings.HasPrefix(low, "summary:") || strings.HasPrefix(low, "tl;dr") {
 			continue
 		}
+		// Normalize common bullet markers; only count this line if it had one.
+		hadMarker := false
+		for _, prefix := range []string{"- ", "* ", "• ", "·"} {
+			if strings.HasPrefix(line, prefix) {
+				line = strings.TrimSpace(line[len(prefix):])
+				hadMarker = true
+				break
+			}
+		}
+		if !hadMarker {
+			continue
+		}
+		sawMarker = true
 		if line == "" {
 			continue
 		}
 		bullets = append(bullets, line)
 	}
-	if len(bullets) == 0 {
+	if !sawMarker || len(bullets) == 0 {
 		// Model produced something but we couldn't parse bullets — pass through.
 		return raw, "<pre>" + html.EscapeString(raw) + "</pre>"
 	}