Architecture: de-hardcode model provider and tool registries

Source: CODEWHALE_0_8_68.md Section 9

Reduce hard-coded registries called out by the architecture context.

Current hard-coded surfaces:
- 81 models in `ModelRegistry::default()`.
- 31 providers in `ProviderKind` enum.
- Approximately 52 tool structs.

Acceptance:
- There is an inventory of which registries are product contracts vs generated/catalog data vs internal implementation.
- Model/provider data moves toward catalog/config driven sources where appropriate.
- Tool registry changes preserve dispatch safety and discoverability.

Start with CODEWHALE_0_8_68.md Section 9, then inspect ModelRegistry::default(), the ProviderKind enum, and the tool registry structs. Inventory which surfaces are product contracts, catalog data, or internal implementation before proposing catalog/config changes; done means the inventory exists and registry changes preserve dispatch safety and discoverability.

Written by the indexing model from the issue text.