fix: resolve shell tabs via OS user shell

[ASRagab]

Summary

• resolve default shell selection via os.userInfo().shell before falling back to process.env.SHELL
• use the shared resolver for both PTY shell tabs and PATH bootstrap in the Electron main process
• add focused tests covering OS shell, env fallback, and /bin/sh fallback behavior

Why

Parallel Code currently falls back to the inherited SHELL environment variable when no explicit shell command is provided. That can cause shell tabs to launch bash even when the user's actual account login shell is zsh (or another shell), depending on how Electron was launched.

Using the OS-reported user shell first makes the default shell selection more faithful and predictable.

Test Plan

• npm run typecheck
• npm test