Split provider setup from workspace bootstrap

[magix022]

Summary

• make init.sh non-interactive so curl -fsSL .../init.sh | bash works
• generate scripts/configure-provider.sh for interactive provider, credential, and CodexLM setup
• document the two-step workshop setup

Verification

• bash -n init.sh
• piped bootstrap from a local curl URL into bash
• interactive OpenAI, CodexLM (auth skipped), and custom LiteLLM provider setup via a pseudo-terminal
• static import of configured starter flows