feat: for codex exec, if PROMPT is not specified, read from stdin if not a TTY (#1178)

This attempts to make `codex exec` more flexible in how the prompt can
be passed:

* as before, it can be passed as a single string argument
* if `-` is passed as the value, the prompt is read from stdin
* if no argument is passed _and stdin is a tty_, prints a warning to
stderr that no prompt was specified an exits non-zero.
* if no argument is passed _and stdin is NOT a tty_, prints `Reading
prompt from stdin...` to stderr to let the user know that Codex will
wait until it reads EOF from stdin to proceed. (You can repro this case
by doing `yes | just exec` since stdin is not a TTY in that case but it
also never reaches EOF).

codex-rs/exec/src/cli.rs

@@ -45,8 +45,10 @@ pub struct Cli {
     #[arg(long = "output-last-message")]
     pub last_message_file: Option<PathBuf>,
 
-    /// Initial instructions for the agent.
-    pub prompt: String,
+    /// Initial instructions for the agent. If not provided as an argument (or
+    /// if `-` is used), instructions are read from stdin.
+    #[arg(value_name = "PROMPT")]
+    pub prompt: Option<String>,
 }
 
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq, ValueEnum)]