Cookbook: Provider-gated AI Calls

Use the stub provider for tests:

let r = std.ai.response_with("stub", "hello")

Use a named provider when the environment is configured:

let r = std.ai.response_with("ollama", "hello")

Use the default provider for deployed environments:

let r = std.ai.response("hello")

Smoke scripts should treat optional live providers as gated:

if [[ -z "${ALEF_OPENAI_API_KEY:-}" ]]; then
  echo "OPENAI_KEY_MISSING: skipped optional live smoke"
  exit 0
fi

Do not hardcode credentials in docs or examples.