refactor(providers): extract provider management to dedicated module

- Create src/agent/providers/ with registry.ts and resolver.ts
- registry.ts: Provider metadata, status checking, login instructions
- resolver.ts: API key resolution, model resolution
- oauth/providers.ts now re-exports from providers/ (deprecated)
- tools.ts: Remove PROVIDER_ALIAS and DEFAULT_MODELS (moved to providers/)
- Update imports in runner.ts and chat.ts

This separates concerns:
- oauth/ only handles OAuth credential reading
- providers/ manages all provider metadata and resolution

src/agent/cli/commands/chat.ts

@@ -17,7 +17,7 @@ import {
   getCurrentProvider,
   getLoginInstructions,
   type ProviderInfo,
-} from "../../oauth/providers.js";
+} from "../../providers/index.js";
 
 type ChatOptions = {
   profile?: string;