fix: remove dependency on expanduser crate (#667)

In putting up https://github.com/openai/codex/pull/665, I discovered
that the `expanduser` crate does not compile on Windows. Looking into
it, we do not seem to need it because we were only using it with a value
that was passed in via a command-line flag, so the shell expands `~` for
us before we see it, anyway. (I changed the type in `Cli` from `String`
to `PathBuf`, to boot.)

If we do need this sort of functionality in the future,
https://docs.rs/shellexpand/latest/shellexpand/fn.tilde.html seems
promising.

codex-rs/repl/src/cli.rs

@@ -56,9 +56,9 @@ pub struct Cli {
 
     /// Record submissions into file as JSON
     #[arg(short = 'S', long)]
-    pub record_submissions: Option<String>,
+    pub record_submissions: Option<PathBuf>,
 
     /// Record events into file as JSON
     #[arg(short = 'E', long)]
-    pub record_events: Option<String>,
+    pub record_events: Option<PathBuf>,
 }