fix(config): update default model to gpt-5-nano

- Updated the default model to gpt-5-nano.
- Adjusted configuration for API compatibility.

The default model in the config has been changed to gpt-5-nano to ensure
compatibility with the latest API features and improvements.

Co-Authored-By: Git Agent <noreply@git-agent.dev>

Author: xesrevinu

src/config/env.ts

@@ -1,7 +1,7 @@
 import { Config, Effect, Option } from "effect";
 
 export const DefaultBaseUrl = "https://api.openai.com/v1";
-export const DefaultModel = "gpt-4o-mini";
+export const DefaultModel = "gpt-5-nano";
 export const DefaultGitignoreBaseUrl = "https://www.toptal.com/developers/gitignore/api";
 
 export const envString = (name: string) => Config.string(name);