feat: stabilize list output with fixed-height preview (5 lines, 300 chars)

Each session always occupies the same number of lines, preventing
visual jitter between watch refreshes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

state.go

@@ -64,7 +64,7 @@ func DetectState(messages []JSONLMessage, now time.Time) SessionState {
 }
 
 // ExtractPreview finds the last assistant text content and returns the first
-// 3 lines, truncated to 200 characters.
+// 5 lines, truncated to 300 characters.
 func ExtractPreview(messages []JSONLMessage) string {
 	for i := len(messages) - 1; i >= 0; i-- {
 		msg := messages[i]
@@ -80,15 +80,18 @@ func ExtractPreview(messages []JSONLMessage) string {
 	return ""
 }
 
+const previewLines = 5
+const previewMaxChars = 300
+
 func truncatePreview(text string) string {
-	lines := strings.SplitN(text, "\n", 4)
-	if len(lines) > 3 {
-		lines = lines[:3]
+	lines := strings.SplitN(text, "\n", previewLines+1)
+	if len(lines) > previewLines {
+		lines = lines[:previewLines]
 	}
 	result := strings.Join(lines, "\n")
 
-	if len(result) > 200 {
-		result = result[:200] + "..."
+	if len(result) > previewMaxChars {
+		result = result[:previewMaxChars] + "..."
 	}
 	return result
 }