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/lib.rs

@@ -1,3 +1,9 @@
+// - In the default output mode, it is paramount that the only thing written to
+//   stdout is the final message (if any).
+// - In --json mode, stdout must be valid JSONL, one event per line.
+// For both modes, any other output must be written to stderr.
+#![deny(clippy::print_stdout)]
+
 mod cli;
 mod event_processor;
 mod event_processor_with_human_output;