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.

browser_cli/async_sdk.py

@@ -84,6 +84,7 @@ class AsyncDecoratorsNS(WorkflowDecoratorsMixin):
     wait: bool = False,
     timeout: float = 30.0,
     background: bool = False,
+    focus: bool = False,
     window: str | None = None,
     group: str | None = None,
     close: bool = False,
@@ -95,6 +96,7 @@ class AsyncDecoratorsNS(WorkflowDecoratorsMixin):
         wait=wait,
         timeout=timeout,
         background=background,
+        focus=focus,
         window=window,
         group=group,
       )