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.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "browser-cli"
-version = "0.12.2"
+version = "0.12.3"
 description = "Control your real running browser from the terminal or Python SDK"
 requires-python = ">=3.10"
 dependencies = [