feat: add Gemini and Bedrock model providers

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.

src/models/index.ts

@@ -1,5 +1,7 @@
 export { AnthropicClient, type AnthropicClientConfig } from './anthropic.js';
 export { OpenAIClient, type OpenAIClientConfig } from './openai.js';
+export { GeminiClient, type GeminiClientConfig } from './gemini.js';
+export { BedrockClient, type BedrockClientConfig } from './bedrock.js';
 export { OllamaClient, type OllamaClientConfig } from './local/index.js';
 export { LlamaCppClient, type LlamaCppClientConfig } from './local/index.js';
 export { ModelRouter, type ModelRouterConfig, type ModelTier } from './router.js';