docs: document register_command() for plugin slash commands (#10671)

* 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

website/docs/user-guide/features/plugins.md

@@ -83,6 +83,7 @@ Project-local plugins under `./.hermes/plugins/` are disabled by default. Enable
 |-----------|-----|
 | Add tools | `ctx.register_tool(name, schema, handler)` |
 | Add hooks | `ctx.register_hook("post_tool_call", callback)` |
+| Add slash commands | `ctx.register_command(name, handler, description)` — adds `/name` in CLI and gateway sessions |
 | Add CLI commands | `ctx.register_cli_command(name, help, setup_fn, handler_fn)` — adds `hermes <plugin> <subcommand>` |
 | Inject messages | `ctx.inject_message(content, role="user")` — see [Injecting Messages](#injecting-messages) |
 | Ship data files | `Path(__file__).parent / "data" / "file.yaml"` |
@@ -117,7 +118,7 @@ Hermes has three kinds of plugins:
 
 | Type | What it does | Selection | Location |
 |------|-------------|-----------|----------|
-| **General plugins** | Add tools, hooks, CLI commands | Multi-select (enable/disable) | `~/.hermes/plugins/` |
+| **General plugins** | Add tools, hooks, slash commands, CLI commands | Multi-select (enable/disable) | `~/.hermes/plugins/` |
 | **Memory providers** | Replace or augment built-in memory | Single-select (one active) | `plugins/memory/` |
 | **Context engines** | Replace the built-in context compressor | Single-select (one active) | `plugins/context_engine/` |