docs: clarify optional Anthropic tool defaults (#5980)

Add a short comment above the quickstart configs to clarify that these
four tools are disabled by default. Keep the runnable example and
documentation in sync.

Validation: pre-commit passed, Python syntax checked, all three
quickstart copies match, and the diff contains only the requested
comment. No runtime behavior changes.


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---
## Summary by cubic
Adds a comment to the Anthropic quickstart examples clarifying that the
browser tools are disabled by default, so readers don't mistake the
example config for default behavior.

<sup>Written for commit 20be69616e9aaa34a665bcfe7934b359197e0580.
Summary will update on new commits.</sup>

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