Document Browserbase browse.sh routing and fallback behavior

## Problem

Browser automation environments may need to route through Browserbase/browse.sh rather than a local browser. A downstream patch adds Browserbase routing notes and fallback guidance to the browse skill docs.

## Expected behavior

The browse skill docs should explain when to use Browserbase/browse.sh routing, how to configure the routing environment, what to verify before assuming browser automation is broken, and fallback behavior when native/local browser automation fails.

## Current downstream patch

A downstream patch updates `browse/SKILL.md` and adds a `browse/references/browserbase-browse-sh-routing.md` reference.