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.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "real-browser-cli"
-version = "0.15.5"
+version = "0.15.6"
 description = "Control your real running browser from the terminal or Python SDK"
 readme = "README.md"
 license = { file = "LICENSE" }