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/sdk/tabs.py

@@ -24,17 +24,19 @@ class TabsNS(Namespace):
     wait: bool = False,
     timeout: float = 30.0,
     background: bool = False,
+    focus: bool = False,
     window: str | None = None,
     group: str | None = None,
   ) -> Tab:
     """Open *url* in a new tab and return a bound :class:`Tab`.
 
     Set ``wait=True`` to block until the page reaches ``readyState=complete``.
+    Pass ``focus=True`` to explicitly bring the created tab/window forward.
     """
     if wait:
-      return self._c.nav.open_wait(url, timeout=timeout, background=background, window=window, group=group)
+      return self._c.nav.open_wait(url, timeout=timeout, background=background, focus=focus, window=window, group=group)
     return self.require_tab(
-      self.command("navigate.open", {"url": url, "background": background, "window": window, "group": group}),
+      self.command("navigate.open", {"url": url, "background": background or not focus, "focus": focus, "window": window, "group": group}),
       "navigate.open returned unexpected data",
     )