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>

display.go

@@ -92,10 +92,12 @@ func DisplaySessionInfos(w io.Writer, sessions []SessionInfo, noColor bool, now
 
 		fmt.Fprintf(w, "[%s] %s%s%s%s%s\n", stateStr, s.Cwd, branch, workspace, label, waiting)
 
-		if s.Preview != "" {
-			lines := strings.Split(s.Preview, "\n")
-			for _, line := range lines {
-				fmt.Fprintf(w, "         %s\n", line)
+		lines := strings.Split(s.Preview, "\n")
+		for i := 0; i < previewLines; i++ {
+			if i < len(lines) && lines[i] != "" {
+				fmt.Fprintf(w, "         %s\n", lines[i])
+			} else {
+				fmt.Fprintln(w)
 			}
 		}
 	}