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/navigate.py

@@ -10,13 +10,13 @@ def nav_group():
 
 @nav_group.command("open")
 @click.argument("url")
-@click.option("--bg", is_flag=True, help="Open in background (no focus)")
+@click.option("--focus", is_flag=True, help="Bring the opened tab/window to the front")
 @click.option("--window", "window_name", default=None, help="Open in named window")
 @click.option("--group", "group_name", default=None, help="Open directly into a tab group (name or ID)")
 @handle_errors
-def cmd_open(url, bg, window_name, group_name):
-    """Open URL in a new tab."""
-    client_from_ctx().nav.open(url, background=bg, window=window_name, group=group_name)
+def cmd_open(url, focus, window_name, group_name):
+    """Open URL in a new tab without stealing focus by default."""
+    client_from_ctx().nav.open(url, focus=focus, window=window_name, group=group_name)
     suffix = ""
     if group_name:
         suffix = f" in group '{group_name}'"
@@ -70,13 +70,13 @@ def cmd_focus(pattern):
 @nav_group.command("open-wait")
 @click.argument("url")
 @click.option("--timeout", type=float, default=30.0, show_default=True, help="Max seconds to wait for load")
-@click.option("--bg", is_flag=True, help="Open in background (no focus)")
+@click.option("--focus", is_flag=True, help="Bring the opened tab/window to the front")
 @click.option("--window", "window_name", default=None, help="Open in named window")
 @click.option("--group", "group_name", default=None, help="Open in tab group")
 @handle_errors
-def cmd_open_wait(url, timeout, bg, window_name, group_name):
+def cmd_open_wait(url, timeout, focus, window_name, group_name):
     """Open URL in a new tab and wait until fully loaded."""
-    tab = client_from_ctx().nav.open_wait(url, timeout=timeout, background=bg, window=window_name, group=group_name)
+    tab = client_from_ctx().nav.open_wait(url, timeout=timeout, focus=focus, window=window_name, group=group_name)
     console.print(f"[green]Loaded:[/green] {url}" + (f" — {tab.title}" if tab.title else ""))
 
 @nav_group.command("wait")