feat(cleanup): default idle-shutdown + keep — stop leaving scratch tabs/groups behind

Agents finish a task and just stop (never calling `close`), so daemons used to
run forever, leaving their per-session scratch tabs + tab group in the user's
Chrome. Two cases now handled:

- Default idle timeout (10 min; AGENT_BROWSER_IDLE_TIMEOUT_MS overrides, 0
  disables). On idle the daemon close()s the tabs IT created → the empty tab
  group is auto-removed by Chrome. Timer resets on every command, so active
  sessions are untouched. Adopted user tabs are never owned, so never closed.
- `keep` — leave the ACTIVE tab for the user: unown it (exempt from
  close/idle) + ask the extension to ungroup it (ABExt.ungroupTab → 0.4.12) so
  it becomes a normal tab. Scratch gets cleaned, deliverable tabs stay.

Also fix two clippy violations in the concurrently-landed #47 viewport code
(manual char comparison + iter().any→contains) that were failing main's CI.

ext 0.4.12: handle ABExt.ungroupTab (chrome.tabs.ungroup). 870 tests pass.

cli/src/native/browser.rs

@@ -1546,6 +1546,13 @@ impl BrowserManager {
             .ok_or_else(|| "No active page".to_string())
     }
 
+    /// Stop owning a tab — drop it from `created_targets` so it survives `close()`
+    /// and idle-shutdown (the agent is leaving it for the user). Returns true if it
+    /// was owned. Used by `keep`.
+    pub fn unown_target(&mut self, target_id: &str) -> bool {
+        self.created_targets.remove(target_id)
+    }
+
     /// Returns true if this manager was connected via CDP (as opposed to local launch).
     pub fn is_cdp_connection(&self) -> bool {
         self.browser_process.is_none()