feat: configurable command name and register toggle

Add a commandName plugin option (default goal, leading slash tolerated)
so the plugin can own e.g. /objective, with all user-facing hints
following the configured name, and a registerCommand:false option that
omits the command hook entirely for programmatic-only setups. Adds
normalizeCommandOptions, parameterizes the /goal hint strings, and adds
unit + command-hook tests. Item 8.2.