refactor(index): extract config loading and handlers to reduce file size

Reduce index.ts from 724 to 458 lines (37% reduction):
- Extract config loading to plugin-config.ts
- Extract ModelCacheState to plugin-state.ts
- Extract config handler to plugin-handlers/config-handler.ts

All 408 tests pass, TypeScript typecheck clean.

🤖 Generated with assistance of OhMyOpenCode (https://github.com/code-yeongyu/oh-my-opencode)

src/plugin-handlers/index.ts

@@ -0,0 +1 @@
+export { createConfigHandler, type ConfigHandlerDeps } from "./config-handler";