Add native GeminiClient using @google/generative-ai SDK and BedrockClient
using @aws-sdk/client-bedrock-runtime. Replace the previous Gemini fallback
(OpenAI-compatible shim) with the real implementation. Add OpenRouter as a
provider option (OpenAI-compatible with custom baseURL). Update model costs,
doctor CLI checks, and client factory tests.
- Extract createClientFromConfig() to dispatch on provider field instead
of hardcoding all tiers as AnthropicClient
- Add fallback/fallbackReason metadata to ChatResponse and ChatStreamEvent
so callers know when a fallback model was used
- Enhance doctor check to report full model stack and warn on missing
API keys for cloud providers
- Log fallback warnings in NativeAgent and display them in TUI
- Support tier names and local_providers entries in fallback_chain
- Add 8 tests for createClientFromConfig covering all provider types