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.

README.md

@@ -138,9 +138,9 @@ Important: profile aliases are browser-instance aliases, not window aliases. Win
 ### Navigation (`nav`)
 
 ```sh
-# Open a URL
+# Open a URL (no focus stealing by default)
 browser-cli nav open https://example.com
-browser-cli nav open https://example.com --bg               # background, no focus
+browser-cli nav open https://example.com --focus            # bring opened tab/window forward
 browser-cli nav open https://example.com --window work      # into a named window
 browser-cli nav open https://example.com --group research   # into a tab group (name or ID)
 
@@ -163,7 +163,7 @@ Each search command opens the search results in your browser using the same flag
 
 ```sh
 browser-cli search google openai api
-browser-cli search brave rust iterators --bg
+browser-cli search brave rust iterators
 browser-cli search ddg tab groups --window work
 browser-cli search youtube browser automation
 browser-cli search yt lo fi