Fix issue #4739: '[Bug]: The agent doesn'"'"'t know its name'

[xingyaoww]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

Explicitly tell the agent "You are OpenHands agent, a helpful AI assistant that can interact with a computer to solve tasks."

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Link of any specific issues this addresses

Fix #4739

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=ghcr.io/all-hands-ai/runtime:296b15e-nikolaik   --name openhands-app-296b15e   ghcr.io/all-hands-ai/runtime:296b15e

[neubig]

I approve, but I think "instructed" might be more clear than "configured" here.