feat: group multi-browser output by source

- Add browser source grouping metadata to SDK-created tabs, groups,
  list results, and aggregate count results.
- Render grouped local/remote browser tables consistently for clients,
  tabs, groups, windows, sessions, and remote status output.
- Document remote control, auth, HTTP gateway usage, and the refreshed
  project structure in the README.
- Add coverage for grouped output and BrowserCounts browser_groups.
- Bump the Python package, extension manifest, and lockfile to 0.15.6.
- Add a just publish helper for building and publishing release artifacts.

uv.lock

@@ -465,7 +465,7 @@ wheels = [
 
 [[package]]
 name = "real-browser-cli"
-version = "0.15.5"
+version = "0.15.6"
 source = { editable = "." }
 dependencies = [
     { name = "click" },