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/event_processor_with_jsonl_output.rs

@@ -428,6 +428,7 @@ impl EventProcessor for EventProcessorWithJsonOutput {
         });
     }
 
+    #[allow(clippy::print_stdout)]
     fn process_event(&mut self, event: Event) -> CodexStatus {
         let aggregated = self.collect_thread_events(&event);
         for conv_event in aggregated {