feat: codex exec writes only the final message to stdout (#4644)

This updates `codex exec` so that, by default, most of the agent's
activity is written to stderr so that only the final agent message is
written to stdout. This makes it easier to pipe `codex exec` into
another tool without extra filtering.

I introduced `#![deny(clippy::print_stdout)]` to help enforce this
change and renamed the `ts_println!()` macro to `ts_msg()` because (1)
it no longer calls `println!()` and (2), `ts_eprintln!()` seemed too
long of a name.

While here, this also adds `-o` as an alias for `--output-last-message`.

Fixes https://github.com/openai/codex/issues/1670

codex-rs/exec/src/cli.rs

@@ -72,7 +72,7 @@ pub struct Cli {
     pub include_plan_tool: bool,
 
     /// Specifies file where the last message from the agent should be written.
-    #[arg(long = "output-last-message")]
+    #[arg(long = "output-last-message", short = 'o', value_name = "FILE")]
     pub last_message_file: Option<PathBuf>,
 
     /// Initial instructions for the agent. If not provided as an argument (or