Clarify codex exec approval help (#16888)

Addresses #13614

Problem: `codex exec --help` implied that `--full-auto` also changed
exec approval mode, even though non-interactive exec stays headless and
does not support interactive approval prompts.

Solution: clarify the `--full-auto` help text so it only describes the
sandbox behavior it actually enables for `codex exec`.

codex-rs/exec/src/cli.rs

@@ -47,7 +47,7 @@ pub struct Cli {
     #[arg(long = "profile", short = 'p')]
     pub config_profile: Option<String>,
 
-    /// Convenience alias for low-friction sandboxed automatic execution (-a on-request, --sandbox workspace-write).
+    /// Convenience alias for low-friction sandboxed automatic execution (--sandbox workspace-write).
     #[arg(long = "full-auto", default_value_t = false, global = true)]
     pub full_auto: bool,