Driving the user's real Chrome should not yank their view around. Now the agent
operates entirely in the background:
- New tabs are created with `background: true` (CreateTargetParams) so opening
one never foregrounds it (the ab-connect extension already used active:false;
this covers the raw-CDP path too).
- Dropped the two AUTO `Page.bringToFront` calls (auto-connect fresh tab, and the
internal active-page switch). The explicit `bringToFront` command is untouched —
surfacing a tab stays opt-in.
- enable_domains now sets `Emulation.setFocusEmulationEnabled(true)` so a
backgrounded agent tab still renders (screenshots work), isn't render-throttled,
and reports document.hasFocus()/visibilityState='visible' — which also removes
the "tab is hidden the whole session" bot tell.
Verified headless: hasFocus=true/visible while backgrounded; click + screenshot
still work. Default behaviour, no flag.