feat(claude): expose hidden fable model

[DongyunZou]

Summary

• Ask Claude subscription accounts to include the hidden fable alias through ANTHROPIC_CUSTOM_MODEL_OPTION.
• Preserve custom model aliases from Claude's model catalogue so hmz passes --model fable.
• Keep API key, gateway, Bedrock, and Vertex catalogues account-specific.
• Add model/TUI regression coverage and documentation.

Existing model caches need one r refresh in the model picker.

Verification

• uv run pre-commit run --all-files
• uv run pytest (1571 passed, 70 skipped)