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

hermes_cli/plugins.py

@@ -23,6 +23,12 @@ Tool registration
 -----------------
 ``PluginContext.register_tool()`` delegates to ``tools.registry.register()``
 so plugin-defined tools appear alongside the built-in tools.
+
+Slash command registration
+--------------------------
+``PluginContext.register_command()`` adds a slash command to the central
+``COMMAND_REGISTRY`` so it appears in /help, autocomplete, and gateway
+dispatch.  Handlers receive the argument string and return a response.
 """
 
 from __future__ import annotations
@@ -95,6 +101,7 @@ class LoadedPlugin:
     module: Optional[types.ModuleType] = None
     tools_registered: List[str] = field(default_factory=list)
     hooks_registered: List[str] = field(default_factory=list)
+    commands_registered: List[str] = field(default_factory=list)
     enabled: bool = False
     error: Optional[str] = None
 
@@ -141,6 +148,45 @@ class PluginContext:
         self._manager._plugin_tool_names.add(name)
         logger.debug("Plugin %s registered tool: %s", self.manifest.name, name)
 
+    # -- command registration ------------------------------------------------
+
+    def register_command(
+        self,
+        name: str,
+        handler: Callable,
+        description: str = "",
+        aliases: tuple[str, ...] = (),
+        args_hint: str = "",
+        cli_only: bool = False,
+        gateway_only: bool = False,
+    ) -> None:
+        """Register a slash command in the central command registry.
+
+        The *handler* is called with a single ``args`` string (everything
+        after the command name) and should return a string to display to the
+        user, or ``None`` for no output.  Async handlers are also supported
+        (they will be awaited in the gateway).
+
+        The command automatically appears in ``/help``, tab-autocomplete,
+        Telegram bot menu, Slack subcommand mapping, and gateway dispatch.
+        """
+        from hermes_cli.commands import CommandDef, register_plugin_command
+
+        cmd_def = CommandDef(
+            name=name,
+            description=description or f"Plugin command: {name}",
+            category="Plugins",
+            aliases=aliases,
+            args_hint=args_hint,
+            cli_only=cli_only,
+            gateway_only=gateway_only,
+        )
+        register_plugin_command(cmd_def)
+        self._manager._plugin_commands[name] = handler
+        for alias in aliases:
+            self._manager._plugin_commands[alias] = handler
+        logger.debug("Plugin %s registered command: /%s", self.manifest.name, name)
+
     # -- hook registration --------------------------------------------------
 
     def register_hook(self, hook_name: str, callback: Callable) -> None:
@@ -172,6 +218,7 @@ class PluginManager:
         self._plugins: Dict[str, LoadedPlugin] = {}
         self._hooks: Dict[str, List[Callable]] = {}
         self._plugin_tool_names: Set[str] = set()
+        self._plugin_commands: Dict[str, Callable] = {}
         self._discovered: bool = False
 
     # -----------------------------------------------------------------------
@@ -325,6 +372,14 @@ class PluginManager:
                         for h in p.hooks_registered
                     }
                 )
+                loaded.commands_registered = [
+                    c for c in self._plugin_commands
+                    if c not in {
+                        n
+                        for name, p in self._plugins.items()
+                        for n in p.commands_registered
+                    }
+                ]
                 loaded.enabled = True
 
         except Exception as exc:
@@ -420,6 +475,7 @@ class PluginManager:
                     "enabled": loaded.enabled,
                     "tools": len(loaded.tools_registered),
                     "hooks": len(loaded.hooks_registered),
+                    "commands": len(loaded.commands_registered),
                     "error": loaded.error,
                 }
             )
@@ -454,3 +510,8 @@ def invoke_hook(hook_name: str, **kwargs: Any) -> None:
 def get_plugin_tool_names() -> Set[str]:
     """Return the set of tool names registered by plugins."""
     return get_plugin_manager()._plugin_tool_names
+
+
+def get_plugin_command_handler(name: str) -> Optional[Callable]:
+    """Return the handler for a plugin-registered slash command, or None."""
+    return get_plugin_manager()._plugin_commands.get(name)