docs: clarify open-source model prompt guidance

[tsubasakong]

Summary

• add a README FAQ entry for the browser-use/bu-30b-a3b-preview model
• clarify that the normal Agent(...) setup still sends the default Browser Use system prompt
• explain that extend_system_message / override_system_message are only needed for intentional prompt customization

Closes #4225

Validation

• git diff --check

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Summary by cubic

Clarifies the README FAQ for the open‑source preview model browser-use/bu-30b-a3b-preview to address #4225.
Confirms that ChatBrowserUse(model='browser-use/bu-30b-a3b-preview') with a standard Agent(...) still uses the default Browser Use system prompt; only use extend_system_message or override_system_message when customizing, setup is unchanged except model=, and hosted bu-* models remain recommended for speed/accuracy.

^{Written for commit ac8671a. Summary will update on new commits.}

[CLAassistant]

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[cubic-dev-ai]

No issues found across 1 file

[sauravpanda]

@tsubasakong can you sign the CLA so that I can merge this PR