Improve error message when API key is missing #86

Description

When a user runs a pipeline without setting the required API key (e.g. OPENAI_API_KEY), the error message could be more helpful — pointing to the docs setup guide and the specific env var needed.

What to do

1. Find where API key validation happens (likely in ondine/adapters/ or ondine/core/)
2. Improve the error message to include:
   • Which env var is missing (e.g. OPENAI_API_KEY)
   • A link to the docs setup page
   • The ondine list-providers command for reference
3. Add a unit test for the improved error message

Acceptance criteria

• Clear, actionable error message when API key is missing
• Unit test passes