Add Anthropic browser toolset driver (#5966)

Claude can use Browser Use as the driver for Anthropic's browser
toolset. This adds all 31 browser actions and Bash from the Browser Use
integration, with local Chromium, Browser Use Cloud, and existing CDP
browser examples.

The driver manages browsers it starts and leaves existing sessions under
application control. Cloud requests negotiate the granted API scope and
retain that version for cleanup. Browser references track document
identity. Uploads and downloads document the boundary between
browser-host paths and SDK-host files; cross-host transfer remains the
application's responsibility.

The root README and integration guide include the selected overview and
tool-call sequence diagrams, editable Excalidraw sources, a Hacker News
quickstart, approval examples, and a retained Cloud terminal/browser
capture. The diagrams place Bash in the Browser Use tool collection and
explain where it executes.

Validation:
- Compatible SDK and Cloud client suite: 21 passed, 1 expected skip;
earlier full compatibility checks also cover SDK versions without the
browser toolset.
- Latest deterministic Cloud audit: 13 checks passed, including approval
denial/error handling, browser-host file reupload, and cleanup.
- Separate Hacker News audit produced reviewed Markdown/JSON and a
screenshot, but its local runner startup and overall cleanup remain
unresolved. This is not reported as a complete passing audit.
- Regression tests cover a detached child holding stdout open, task
cancellation, Cloud cleanup through the selected API version, and scope
renegotiation on new creation.
- Pre-commit, Python snippet parsing, exact docs/quickstart comparison,
SVG/Excalidraw validation, and desktop/mobile documentation rendering
passed.

This PR contains Browser Use code only; it does not vendor or publish
Anthropic's SDK. The exact public Anthropic browser-toolset package
smoke remains outstanding, and the documentation states the required
APIs explicitly.