fix: make navigation no-focus by default

- Change nav open and open-wait to avoid activating newly created tabs unless
  --focus is explicitly requested.
- Send background=true for default opens so older or remote extensions also
  avoid stealing focus even if they ignore the new focus flag.
- Remove the redundant --bg flag from navigation and search CLI commands now
  that no-focus/background behavior is the default.
- Thread focus support through the sync SDK, async SDK, tab helpers, and
  workflow decorators.
- Update README and demo usage to document the new default and --focus opt-in.
- Bump package and extension metadata to 0.12.3.
- Add regression coverage for CLI help, wire payloads, and extension behavior.

tests/test_extension_error_page_handling.py

@@ -111,10 +111,12 @@ def test_large_extension_operations_yield_between_batches():
     assert "perf.set_profile" in perf
     assert "__background" in connection
 
-def test_tab_activation_and_merge_do_not_steal_audible_video_window():
+def test_tab_activation_open_and_merge_do_not_steal_audible_video_window():
     tabs = (ROOT / "extension" / "src" / "commands" / "tabs.ts").read_text()
+    navigation = (ROOT / "extension" / "src" / "commands" / "navigation.ts").read_text()
 
     assert "await chrome.windows.update(tab.windowId, { focused: true });" not in tabs
+    assert "active: Boolean(focus) && !background" in navigation
     assert "windowHasAudibleTabs" in tabs
     assert "!this.windowHasAudibleTabs(w)" in tabs
     assert "skippedAudibleWindows" in tabs