feat(ai): risk-based model routing and fallback

[devops-thiago]

Problem Statement

Every review uses a single configured model. For cost efficiency, low-risk diffs (small, docs-only, generated) could be reviewed by a cheaper model, escalating to a stronger model only for larger or higher-risk changes.

Proposed Solution

Add optional risk-based model routing: classify the diff (size, paths, signals) and select among configured models, with fallback to a secondary model on provider error or rate limit. Make routing rules configurable; default to single-model behavior. Pairs with multi-provider support and per-PR spend caps.

Alternatives Considered

• Single model always — simplest, but leaves cost savings on the table.

Priority

Nice to have

Code of Conduct

• I have searched for existing issues and this is not a duplicate.