Complete the half-built plugin slash command system. The dispatch
code in cli.py and gateway/run.py already called
get_plugin_command_handler() but the registration side was never
implemented.
Changes:
- Add register_command() to PluginContext — stores handler,
description, and plugin name; normalizes names; rejects conflicts
with built-in commands
- Add _plugin_commands dict to PluginManager
- Add commands_registered tracking on LoadedPlugin
- Add get_plugin_command_handler() and get_plugin_commands()
module-level convenience functions
- Fix commands.py to use actual plugin description in Telegram
bot menu (was hardcoded 'Plugin command')
- Add plugin commands to SlashCommandCompleter autocomplete
- Show command count in /plugins display
- 12 new tests covering registration, conflict detection,
normalization, handler dispatch, and introspection
Closes#10495
498b995c13