config: document and validate source files

[son-gladyso]

Goal

Add a small schema or validation reference for config/sources.example.json and document the most useful validation errors.

Acceptance criteria

• Required fields and relative-path behavior are documented.
• Invalid entries produce an actionable local error.
• Existing valid configurations remain compatible.
• Add synthetic tests for at least one valid and three invalid configurations.

Good first contribution

A focused configuration and test improvement; no third-party source integration is needed.