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-state.ts

@@ -0,0 +1,30 @@
+export interface ModelCacheState {
+  modelContextLimitsCache: Map<string, number>;
+  anthropicContext1MEnabled: boolean;
+}
+
+export function createModelCacheState(): ModelCacheState {
+  return {
+    modelContextLimitsCache: new Map<string, number>(),
+    anthropicContext1MEnabled: false,
+  };
+}
+
+export function getModelLimit(
+  state: ModelCacheState,
+  providerID: string,
+  modelID: string
+): number | undefined {
+  const key = `${providerID}/${modelID}`;
+  const cached = state.modelContextLimitsCache.get(key);
+  if (cached) return cached;
+
+  if (
+    providerID === "anthropic" &&
+    state.anthropicContext1MEnabled &&
+    modelID.includes("sonnet")
+  ) {
+    return 1_000_000;
+  }
+  return undefined;
+}