Adds bu-2-0-mini-preview as an accepted model id and makes it the
constructor default, so a bare ChatBrowserUse() now routes there.
bu-2-0 is unchanged: still accepted, still documented as the premium
option, and bu-latest still resolves to it. Keeping 'latest' on the
stable line means existing callers pinned to that alias do not silently
move onto a preview model; only the bare-constructor default moves.
Pricing is registered alongside the model so cost tracking does not
silently report $0 for what is now the default. This model has no cache
discount, so cached reads bill at the input rate.
Examples, README and the model reference are updated to the new default.
The model reference also claimed bu-latest resolved to bu-1-0, which has
not been true since bu-2-0 shipped; corrected here.
Saurav Panda