feat(plugins): add slash command registration for plugins (#2359)

Plugins can now register slash commands via ctx.register_command()
in their register() function. Commands automatically appear in:
- /help and COMMANDS_BY_CATEGORY (under 'Plugins' category)
- Tab autocomplete in CLI
- Telegram bot menu
- Slack subcommand mapping
- Gateway dispatch

Handler signature: handler(args: str) -> str | None
Async handlers are supported in gateway context.

Changes:
- commands.py: add register_plugin_command() and rebuild_lookups()
- plugins.py: add register_command() to PluginContext, track in
  PluginManager._plugin_commands and LoadedPlugin.commands_registered
- cli.py: dispatch plugin commands in process_command()
- gateway/run.py: dispatch plugin commands before skill commands
- tests: 5 new tests for registration, help, tracking, handler, gateway
- docs: update plugins feature page and build guide

website/docs/guides/build-a-hermes-plugin.md

@@ -232,6 +232,7 @@ def register(ctx):
 - Called exactly once at startup
 - `ctx.register_tool()` puts your tool in the registry — the model sees it immediately
 - `ctx.register_hook()` subscribes to lifecycle events
+- `ctx.register_command()` adds a slash command to `/help`, autocomplete, and gateway dispatch
 - If this function crashes, the plugin is disabled but Hermes continues fine
 
 ## Step 6: Test it