chore(config): pin @types/bun, tighten TypeScript strictness, and refactor config logic

- Pin `@types/bun` to `^1.3.14` for reproducible builds instead of using `latest`
- Update `tsconfig.json` with stricter type-checking flags and reorganize compiler options
- Refactor `initConfig` to DRY up the model default logic and add a null guard before process exit

testRender.ts

@@ -1,4 +1,4 @@
-import { renderMarkdown } from "./src/render/markdown.ts";
+import { renderMarkdown } from "./src/render/markdown";
 
 const text = await Bun.file("README.md").text();