feat: auto-approve during login by default

[Saturate]

Summary

mitid login now handles both login and approval in a single command. No more two-terminal dance.

• Resolves the identity and starts approve() in the background before calling login()
• The approve polls until the login flow triggers the push, then approves it
• Use --no-approve to opt out (e.g. when approving from a different machine)
• Updated guide and README

Before

# Terminal 1
mitid login myuser https://service.example.com/login
# Terminal 2
mitid approve myuser

After

mitid login myuser https://service.example.com/login