Revert "Revert "Overhaul shell detection and centralize command generation for unified exec"" (#6607)

Reverts openai/codex#6606

codex-rs/core/src/environment_context.rs

@@ -329,7 +329,6 @@ mod tests {
             Some(workspace_write_policy(vec!["/repo"], false)),
             Some(Shell::Bash(BashShell {
                 shell_path: "/bin/bash".into(),
-                bashrc_path: "/home/user/.bashrc".into(),
             })),
         );
         let context2 = EnvironmentContext::new(
@@ -338,7 +337,6 @@ mod tests {
             Some(workspace_write_policy(vec!["/repo"], false)),
             Some(Shell::Zsh(ZshShell {
                 shell_path: "/bin/zsh".into(),
-                zshrc_path: "/home/user/.zshrc".into(),
             })),
         );