Format large numbers in a more readable way. (#2046)

- In the bottom line of the TUI, print the number of tokens to 3 sigfigs
  with an SI suffix, e.g. "1.23K".
- Elsewhere where we print a number, I figure it's worthwhile to print
  the exact number, because e.g. it's a summary of your session. Here we print
  the numbers comma-separated.

codex-rs/protocol/src/lib.rs

@@ -3,6 +3,7 @@ pub mod custom_prompts;
 pub mod mcp_protocol;
 pub mod message_history;
 pub mod models;
+pub mod num_format;
 pub mod parse_command;
 pub mod plan_tool;
 pub mod protocol;