feat(stealth): silent operation — never steal the user's foreground tab

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.

cli/src/native/cdp/types.rs

@@ -153,6 +153,12 @@ pub struct CreateTargetParams {
     /// endpoint never receives an unknown parameter.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub agent_group: Option<String>,
+    /// Create the tab in the background so opening it never steals the user's
+    /// foreground tab (silent operation). Standard CDP param; the ab-connect
+    /// extension creates its tabs `active: false` regardless, so this only
+    /// affects the raw-CDP (no extension) path.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub background: Option<bool>,
 }
 
 #[derive(Debug, Deserialize)]