docs: add missing docs

ref: N25B-115

src/control_backend/core/agent_system.py

@@ -22,10 +22,22 @@ class AgentDirectory:
 
     @staticmethod
     def register(name: str, agent: "BaseAgent"):
+        """
+        Registers an agent instance with a unique name.
+
+        :param name: The name of the agent.
+        :param agent: The :class:`BaseAgent` instance.
+        """
         _agent_directory[name] = agent
 
     @staticmethod
     def get(name: str) -> "BaseAgent | None":
+        """
+        Retrieves a registered agent instance by name.
+
+        :param name: The name of the agent to retrieve.
+        :return: The :class:`BaseAgent` instance, or None if not found.
+        """
         return _agent_directory.get(name)