feat: update launch screen (#1881)

- Updates the launch screen to:
  ```
  >_ You are using OpenAI Codex in ~/code/codex/codex-rs
  
   Try one of the following commands to get started:
  
   1. /init - Create an AGENTS.md file with instructions for Codex
   2. /status - Show current session configuration and token usage
   3. /compact - Compact the chat history
   4. /new - Start a new chat
   ```
- These aren't the perfect commands, but as more land soon we can
update.
- We should also add logic later to make /init only show when there's no
existing AGENTS.md.
- Majorly need to iterate on copy.

<img width="905" height="769" alt="image"
src="https://github.com/user-attachments/assets/5912939e-fb0e-4e76-94ff-785261e2d6ee"
/>

codex-rs/tui/src/slash_command.rs

@@ -27,7 +27,7 @@ impl SlashCommand {
     pub fn description(self) -> &'static str {
         match self {
             SlashCommand::New => "Start a new chat",
-            SlashCommand::Init => "Create an AGENTS.md file with instructions for Codex.",
+            SlashCommand::Init => "Create an AGENTS.md file with instructions for Codex",
             SlashCommand::Compact => "Compact the chat history",
             SlashCommand::Quit => "Exit the application",
             SlashCommand::Diff => "Show git diff (including untracked files)",