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/messages-transform.ts

@@ -0,0 +1,24 @@
+import type { Message, Part } from "@opencode-ai/sdk"
+
+import type { CreatedHooks } from "../create-hooks"
+
+type MessageWithParts = {
+  info: Message
+  parts: Part[]
+}
+
+type MessagesTransformOutput = { messages: MessageWithParts[] }
+
+export function createMessagesTransformHandler(args: {
+  hooks: CreatedHooks
+}): (input: Record<string, never>, output: MessagesTransformOutput) => Promise<void> {
+  return async (input, output): Promise<void> => {
+    await args.hooks.contextInjectorMessagesTransform?.[
+      "experimental.chat.messages.transform"
+    ]?.(input, output)
+
+    await args.hooks.thinkingBlockValidator?.[
+      "experimental.chat.messages.transform"
+    ]?.(input, output)
+  }
+}