refactor(core): split index.ts and config-handler.ts into focused modules

Main entry point:
- create-hooks.ts, create-tools.ts, create-managers.ts
- plugin-interface.ts: plugin interface types
- plugin/ directory: plugin lifecycle modules

Config handler:
- agent-config-handler.ts, command-config-handler.ts
- tool-config-handler.ts, mcp-config-handler.ts
- provider-config-handler.ts, category-config-resolver.ts
- agent-priority-order.ts, prometheus-agent-config-builder.ts
- plugin-components-loader.ts

src/plugin-handlers/mcp-config-handler.ts

@@ -0,0 +1,21 @@
+import type { OhMyOpenCodeConfig } from "../config";
+import { loadMcpConfigs } from "../features/claude-code-mcp-loader";
+import { createBuiltinMcps } from "../mcp";
+import type { PluginComponents } from "./plugin-components-loader";
+
+export async function applyMcpConfig(params: {
+  config: Record<string, unknown>;
+  pluginConfig: OhMyOpenCodeConfig;
+  pluginComponents: PluginComponents;
+}): Promise<void> {
+  const mcpResult = params.pluginConfig.claude_code?.mcp ?? true
+    ? await loadMcpConfigs()
+    : { servers: {} };
+
+  params.config.mcp = {
+    ...createBuiltinMcps(params.pluginConfig.disabled_mcps, params.pluginConfig),
+    ...(params.config.mcp as Record<string, unknown>),
+    ...mcpResult.servers,
+    ...params.pluginComponents.mcpServers,
+  };
+}