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_api.py

@@ -160,7 +160,7 @@ class TestNavigation:
     b.nav.open("https://example.com")
     mock_send.assert_called_once_with(
       "navigate.open",
-      {"url": "https://example.com", "background": False, "window": None, "group": None},
+      {"url": "https://example.com", "background": True, "focus": False, "window": None, "group": None},
       profile=None, remote=None, key=None,
     )
 
@@ -173,6 +173,10 @@ class TestNavigation:
     b.nav.open("https://x.com", group="Work")
     assert mock_send.call_args[0][1]["group"] == "Work"
 
+  def test_open_focus_is_explicit(self, b, mock_send):
+    b.nav.open("https://example.com", focus=True)
+    assert mock_send.call_args[0][1]["focus"] is True
+
   def test_tabs_open_returns_bound_tab(self, b, mock_send):
     mock_send.return_value = {"id": 123, "url": "https://example.com"}
 
@@ -183,7 +187,7 @@ class TestNavigation:
     assert tab._browser is b
     mock_send.assert_called_once_with(
       "navigate.open",
-      {"url": "https://example.com", "background": True, "window": None, "group": None},
+      {"url": "https://example.com", "background": True, "focus": False, "window": None, "group": None},
       profile=None,
       remote=None,
       key=None,
@@ -197,7 +201,7 @@ class TestNavigation:
     assert tab.id == 10
     mock_send.assert_called_once_with(
       "navigate.open_wait",
-      {"url": "https://example.com", "timeout": 1500, "background": False, "window": None, "group": None},
+      {"url": "https://example.com", "timeout": 1500, "background": True, "focus": False, "window": None, "group": None},
       profile=None,
       remote=None,
       key=None,
@@ -1031,7 +1035,7 @@ class TestSDKDecorators:
     assert mock_send.mock_calls == [
       call(
         "navigate.open_wait",
-        {"url": "https://example.com", "timeout": 1500, "background": False, "window": None, "group": None},
+        {"url": "https://example.com", "timeout": 1500, "background": True, "focus": False, "window": None, "group": None},
         profile=None,
         remote=None,
         key=None,
@@ -1190,7 +1194,7 @@ class TestAsyncBrowserCLI:
       assert mock_send_async.mock_calls == [
         call(
           "navigate.open",
-          {"url": "https://example.com", "background": False, "window": None, "group": None},
+          {"url": "https://example.com", "background": True, "focus": False, "window": None, "group": None},
           profile=None,
           remote=None,
           key=None,