fix: prefer the macOS login shell for new sessions

[samwei12]

Summary

Fixes the macOS case where new terminal sessions start in bash even when the user's real login shell is zsh.

Closes #2.

Root cause

The plugin currently resolves the shell from process.env.SHELL || '/bin/bash'.

CloudCLI starts plugin backend processes with a deliberately restricted environment and does not pass through SHELL, which appears intentional and should stay that way for safety. For the terminal plugin specifically, that causes a fallback to /bin/bash on macOS even when the real login shell is zsh.

What changed

• only on darwin, if process.env.SHELL is missing or still /bin/bash, fall back to os.userInfo().shell
• preserve existing behavior on Windows
• preserve existing process.env.SHELL behavior on non-macOS platforms
• pass the resolved shell into the PTY child env as SHELL so the shell session is internally consistent

Verification

• npm run build
• validated darwin stale-bash fallback to /bin/zsh
• validated explicit env shell remains respected
• validated non-darwin keeps existing behavior
• validated fallback to env.SHELL || /bin/bash when os.userInfo() is unavailable