tui: standardize tree prefix glyphs to └ (#2274)

Replace mixed `⎿` and `L` prefixes with `└` in TUI rendering.

<img width="454" height="659" alt="Screenshot 2025-08-13 at 4 02 03 PM"
src="https://github.com/user-attachments/assets/61c9c7da-830b-4040-bb79-a91be90870ca"
/>

codex-rs/tui/src/diff_render.rs

@@ -143,7 +143,7 @@ pub(crate) fn create_diff_summary(
         spans.push(RtSpan::raw(")"));
 
         let mut line = RtLine::from(spans);
-        let prefix = if idx == 0 { "  ⎿ " } else { "    " };
+        let prefix = if idx == 0 { "  └ " } else { "    " };
         line.spans.insert(0, prefix.into());
         line.spans
             .iter_mut()