hide CoT by default; show headers in status indicator (#2316)

Plan is for full CoT summaries to be visible in a "transcript view" when
we implement that, but for now they're hidden.


https://github.com/user-attachments/assets/e8a1b0ef-8f2a-48ff-9625-9c3c67d92cdb

codex-rs/tui/src/shimmer.rs

@@ -22,7 +22,7 @@ pub(crate) fn shimmer_spans(text: &str) -> Vec<Span<'static>> {
     // Use time-based sweep synchronized to process start.
     let padding = 10usize;
     let period = chars.len() + padding * 2;
-    let sweep_seconds = 2.5f32;
+    let sweep_seconds = 2.0f32;
     let pos_f =
         (elapsed_since_start().as_secs_f32() % sweep_seconds) / sweep_seconds * (period as f32);
     let pos = pos_f as usize;