feat: implement register_command() on plugin context (#10626)

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

cli.py

@@ -5484,7 +5484,8 @@ class HermesCLI:
                         version = f" v{p['version']}" if p["version"] else ""
                         tools = f"{p['tools']} tools" if p["tools"] else ""
                         hooks = f"{p['hooks']} hooks" if p["hooks"] else ""
-                        parts = [x for x in [tools, hooks] if x]
+                        commands = f"{p['commands']} commands" if p.get("commands") else ""
+                        parts = [x for x in [tools, hooks, commands] if x]
                         detail = f" ({', '.join(parts)})" if parts else ""
                         error = f" — {p['error']}" if p["error"] else ""
                         print(f"  {status} {p['name']}{version}{detail}{error}")