Expand VIBE.md with frontend setup instructions

Added instructions for using the OpenHands typescript-client to build a custom frontend for agent interactions.

Author: rbren

VIBE.md

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+# Vibecoding with the OpenHands typescript-client
+
+This client is great for vibecoding small custom frontends that drive agents. Below are some instructions
+you can pass to an agent to bootstrap a new repo.
+
+> You are working with @openhands/typescript-client to build a custom frontend for working with
+> agents that can write and run code. Clone github.com/openhands/typescript-client and investigate the API.
+>
+> To start this frontend, create a basic React application. On the homepage should be a "settings" button, which
+> opens a modal. These four fields should be settable:
+> * AgentServer URL
+> * AgentServer API Key
+> * LLM model
+> * LLM key
+> In the modal, show whether the AgentServer is connected by trying to access http://174.138.76.92:8000/api/conversations/count, and make sure it responds 2xx
+>
+> Store all this in localStorage. If no settings are found on page load, open the modal by default
+>
+> If the settings are found and working, start a new conversation using the typescript-client. Show a message box on the homepage which allows sending messages
+> to the agent, and show the most recent message back from the agent below that
+>
+> Set up GitHub actions to build, test, and lint all this
+