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.

browser_cli/models.py

@@ -31,6 +31,7 @@ class BrowserCounts:
   """Aggregated per-browser counts returned in implicit multi-browser mode."""
   total: int
   by_browser: dict[str, int]
+  browser_groups: dict[str, str] = field(default_factory=dict)
 
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