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/commands/search.py

@@ -63,13 +63,12 @@ def search_group():
 def _build_command(engine_key: str, help_text: str) -> click.Command:
     @click.command(engine_key, help=help_text)
     @click.argument("query", nargs=-1, required=True)
-    @click.option("--bg", is_flag=True, help="Open in background (no focus)")
     @click.option("--window", "window", default=None, help="Open in named window")
     @click.option("--group", "group", default=None, help="Open in tab group (name or ID)")
     @handle_errors
-    def _cmd(query, bg, window, group):
+    def _cmd(query, window, group):
         terms = " ".join(query)
-        client_from_ctx().nav.search(engine_key, terms, background=bg, window=window, group=group)
+        client_from_ctx().nav.search(engine_key, terms, window=window, group=group)
         suffix = f" in group '{group}'" if group else (f" in window '{window}'" if window else "")
         display = _DISPLAY_NAMES.get(engine_key, engine_key.capitalize())
         console.print(f"[green]Searching[/green] [cyan]{display}[/cyan]: {terms}{suffix}")