fix: add /browser to COMMAND_REGISTRY so it shows in help and autocomplete

The /browser command handler existed in cli.py but was never added to
COMMAND_REGISTRY after the centralized command registry refactor. This
meant:
- /browser didn't appear in /help
- No tab-completion or subcommand suggestions
- Dispatch used _base_word fallback instead of canonical resolution

Added CommandDef with connect/disconnect/status subcommands and
switched dispatch to use canonical instead of _base_word.

cli.py

@@ -3560,7 +3560,7 @@ class HermesCLI:
         elif canonical == "reload-mcp":
             with self._busy_command(self._slow_command_status(cmd_original)):
                 self._reload_mcp()
-        elif _base_word == "browser":
+        elif canonical == "browser":
             self._handle_browser_command(cmd_original)
         elif canonical == "plugins":
             try: