After connect+grouping worked, follow-up eval/get-title/screenshot drifted to a
foreign tab: on a shared browser, Target.targetCreated events for tabs the user
or OTHER agent sessions open stream in and are drained on every command. The
drain path routed them through add_page(), which sets active_page_index to the
new page — so the session's active tab silently jumped to a foreign tab and its
commands landed there.
Add BrowserManager::add_background_page() (push without touching active, dedup by
target_id) and use it in the event-drain path. Explicit opens (tab new, the
add-and-switch paths) keep using add_page() and still focus the new tab.
Closes the last gap in concurrent multi-agent: each session now drives its OWN
tab regardless of other sessions'/the user's tab activity.
leeguooooo
06c75af46a