Add Claude Sonnet 4.6 model + deprecated 4.5 fallback resolver

- Phase 1: Add claude-sonnet-4-6 model definition, alias, docs, and test entries
- Task 01: Add MODEL_FALLBACKS map for 4.5→4.6 redirects with deprecation warnings
- Task 01: Add DEPRECATED_MODELS set (10 entries) covering all legacy alias keys
- Task 01: Wire logToast deprecation warning in resolveModelWithTier
- Task 01: Update model-resolver tests (44/44 pass) with 4.6 fixtures and fallback tests