Extract reusable Bash and Zsh startup scripts (#45749)

Expose `shell_startup_script` from `codex-shell-command` and use it for
interactive shell snapshot capture, preserving the existing startup scripts.
The helper returns an empty script for other shell types; callers remain
responsible for login startup flags and shell lifecycle.

GitOrigin-RevId: 8cb64c02c08140d2676e67b9db06c9a77649b3eb

codex-rs/shell-command/src/lib.rs

@@ -2,6 +2,9 @@
 
 pub mod shell_detect;
 pub mod shell_snapshot;
+mod startup;
+
+pub use startup::shell_startup_script;
 
 pub mod bash;
 pub(crate) mod command_safety;