Implement world model extraction after onboarding

[EightRice]

The onboarding conversation completes and saves to onboarding.yaml, but the world model extraction is never triggered.

What needs to happen:

1. Call world_model_manager.build_from_onboarding() at the end of onboarding
2. Implement actual deviation extraction using Claude (currently a stub)

Expected output:

• ~/.chevin/world_model.json with structured deviations (epistemic, motivational, relational, etc.)
• ~/.chevin/sources/ with source documents