Use consistent TUI input placeholders (#37360)

## What changed

- Show `Ask Codex to do anything` in the main conversation input.
- Show `Ask a follow-up question` in side conversation inputs.
- Remove randomized example prompt selection and update the side conversation snapshots.

GitOrigin-RevId: 2f6d310cf80bfa9743b7695621892a821eda4a18

codex-rs/tui/src/chatwidget.rs

@@ -165,7 +165,6 @@ use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
 use crossterm::event::KeyEventKind;
 use crossterm::event::KeyModifiers;
-use rand::Rng;
 use ratatui::buffer::Buffer;
 use ratatui::layout::Rect;
 use ratatui::style::Modifier;
@@ -1977,22 +1976,8 @@ impl Drop for ChatWidget {
     }
 }
 
-const PLACEHOLDERS: [&str; 8] = [
-    "Explain this codebase",
-    "Summarize recent commits",
-    "Implement {feature}",
-    "Find and fix a bug in @filename",
-    "Write tests for @filename",
-    "Improve documentation in @filename",
-    "Run /review on my current changes",
-    "Use /skills to list available skills",
-];
-
-const SIDE_PLACEHOLDERS: [&str; 3] = [
-    "Check recently modified functions for compatibility",
-    "How many files have been modified?",
-    "Will this algorithm scale well?",
-];
+const PLACEHOLDER: &str = "Ask Codex to do anything";
+const SIDE_PLACEHOLDER: &str = "Ask a follow-up question";
 
 // Extract the first bold (Markdown) element in the form **...** from `s`.
 // Returns the inner text if found; otherwise `None`.