feat: add look_at tool and multimodal-looker agent

Add a new tool and agent for analyzing media files (PDFs, images, diagrams)
that require visual interpretation beyond raw text.

- Add `multimodal-looker` agent using Gemini 2.5 Flash model
- Add `look_at` tool that spawns multimodal-looker sessions
- Restrict multimodal-looker from calling task/call_omo_agent/look_at tools

Inspired by Sourcegraph Ampcode's look_at tool design.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/tools/index.ts

@@ -34,6 +34,7 @@ import type { BackgroundManager } from "../features/background-agent"
 type OpencodeClient = PluginInput["client"]
 
 export { createCallOmoAgent } from "./call-omo-agent"
+export { createLookAt } from "./look-at"
 
 export function createBackgroundTools(manager: BackgroundManager, client: OpencodeClient) {
   return {