feat(exec): add --fork option to codex exec

Add non-interactive session forking to codex exec and keep the CLI/help/tests aligned with the interactive fork behavior.

codex-rs/exec/tests/suite/mod.rs

@@ -3,6 +3,7 @@ mod add_dir;
 mod apply_patch;
 mod auth_env;
 mod ephemeral;
+mod fork;
 mod mcp_required_exit;
 mod originator;
 mod output_schema;