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.

examples/demo.sh

@@ -43,8 +43,8 @@ pause
 header "3/8 · Create 'research' group and open URLs into it"
 $CLI groups create research
 echo ""
-$CLI nav open https://example.com   --group research --bg
-$CLI nav open https://wikipedia.org --group research --bg
+$CLI nav open https://example.com   --group research
+$CLI nav open https://wikipedia.org --group research
 echo ""
 echo "  Tabs are now open inside the 'research' group in your browser."
 pause