chore: rename codex-command to codex-shell-command (#11378)

This addresses some post-merge feedback on
https://github.com/openai/codex/pull/11361:

- crate rename
- reuse `detect_shell_type()` utility

codex-rs/core/src/shell_detect.rs

@@ -0,0 +1,24 @@
+use crate::shell::ShellType;
+use std::path::Path;
+use std::path::PathBuf;
+
+pub(crate) fn detect_shell_type(shell_path: &PathBuf) -> Option<ShellType> {
+    match shell_path.as_os_str().to_str() {
+        Some("zsh") => Some(ShellType::Zsh),
+        Some("sh") => Some(ShellType::Sh),
+        Some("cmd") => Some(ShellType::Cmd),
+        Some("bash") => Some(ShellType::Bash),
+        Some("pwsh") => Some(ShellType::PowerShell),
+        Some("powershell") => Some(ShellType::PowerShell),
+        _ => {
+            let shell_name = shell_path.file_stem();
+            if let Some(shell_name) = shell_name {
+                let shell_name_path = Path::new(shell_name);
+                if shell_name_path != Path::new(shell_path) {
+                    return detect_shell_type(&shell_name_path.to_path_buf());
+                }
+            }
+            None
+        }
+    }
+}