fix: fail fast on auth errors in non-interactive and agent contexts (#1537)

- Extract shouldAutoLogin(getEnv, stdinIsTerminal) helper in pkg/cmd so
  both login.go and root.go share the same logic: return true only when
  stdin is a real TTY and no AI agent env var is set
- root.go isLoginRequiredError: replace bare DetectAIAgent check with
  shouldAutoLogin, covering CI/headless environments (non-TTY stdin) in
  addition to agent contexts; print actionable message to stderr and exit
  instead of blocking on a browser flow
- login.go runLoginCmd: replace inline TTY+agent check with shouldAutoLogin
- Redirect all auth error output to stderr consistently
- Add TestShouldAutoLogin covering all four agent/no-agent × TTY/non-TTY cases


Committed-By-Agent: claude

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>