tui: • Working, 100% context dim (#4629)

- add a `•` before the "Working" shimmer
- make the percentage in "X% context left" dim instead of bold

<img width="751" height="480" alt="Screenshot 2025-10-02 at 2 29 57 PM"
src="https://github.com/user-attachments/assets/cf3e771f-ddb3-48f4-babe-1eaf1f0c2959"
/>

codex-rs/tui/src/history_cell.rs

@@ -994,7 +994,7 @@ impl HistoryCell for PlanUpdateCell {
                 indented_lines.extend(render_step(status, step));
             }
         }
-        lines.extend(prefix_lines(indented_lines, "  └ ".into(), "    ".into()));
+        lines.extend(prefix_lines(indented_lines, "  └ ".dim(), "    ".into()));
 
         lines
     }