* feat: implement register_command() on plugin context
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
* docs: add register_command() to plugin guides
- Build a Plugin guide: new 'Register slash commands' section with
full API reference, comparison table vs register_cli_command(),
sync/async examples, and conflict protection docs
- Features/Plugins page: add slash commands to capabilities table
and plugin types summary
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