feat(agent): add AIAgent.close() for subprocess cleanup

Add a close() method to AIAgent that acts as a single entry point for
releasing all resources held by an agent instance. This prevents zombie
process accumulation on long-running gateway deployments by explicitly
cleaning up:

- Background processes tracked in ProcessRegistry
- Terminal sandbox environments
- Browser daemon sessions
- Active child agents (subagent delegation)
- OpenAI/httpx client connections

Each cleanup step is independently guarded so a failure in one does not
prevent the rest. The method is idempotent and safe to call multiple
times.

Also simplifies the background review cleanup to use close() instead
of manually closing the OpenAI client.

Ref: #7131

run_agent.py

@@ -1977,19 +1977,14 @@ class AIAgent:
             except Exception as e:
                 logger.debug("Background memory/skill review failed: %s", e)
             finally:
-                # Explicitly close the OpenAI/httpx client so GC doesn't
-                # try to clean it up on a dead asyncio event loop (which
-                # produces "Event loop is closed" errors in the terminal).
+                # Close all resources (httpx client, subprocesses, etc.) so
+                # GC doesn't try to clean them up on a dead asyncio event
+                # loop (which produces "Event loop is closed" errors).
                 if review_agent is not None:
-                    client = getattr(review_agent, "client", None)
-                    if client is not None:
-                        try:
-                            review_agent._close_openai_client(
-                                client, reason="bg_review_done", shared=True
-                            )
-                            review_agent.client = None
-                        except Exception:
-                            pass
+                    try:
+                        review_agent.close()
+                    except Exception:
+                        pass
 
         t = threading.Thread(target=_run_review, daemon=True, name="bg-review")
         t.start()
@@ -2729,6 +2724,64 @@ class AIAgent:
             except Exception:
                 pass
     
+    def close(self) -> None:
+        """Release all resources held by this agent instance.
+
+        Cleans up subprocess resources that would otherwise become orphans:
+        - Background processes tracked in ProcessRegistry
+        - Terminal sandbox environments
+        - Browser daemon sessions
+        - Active child agents (subagent delegation)
+        - OpenAI/httpx client connections
+
+        Safe to call multiple times (idempotent).  Each cleanup step is
+        independently guarded so a failure in one does not prevent the rest.
+        """
+        task_id = getattr(self, "session_id", None) or ""
+
+        # 1. Kill background processes for this task
+        try:
+            from tools.process_registry import process_registry
+            process_registry.kill_all(task_id=task_id)
+        except Exception:
+            pass
+
+        # 2. Clean terminal sandbox environments
+        try:
+            from tools.terminal_tool import cleanup_vm
+            cleanup_vm(task_id)
+        except Exception:
+            pass
+
+        # 3. Clean browser daemon sessions
+        try:
+            from tools.browser_tool import cleanup_browser
+            cleanup_browser(task_id)
+        except Exception:
+            pass
+
+        # 4. Close active child agents
+        try:
+            with self._active_children_lock:
+                children = list(self._active_children)
+                self._active_children.clear()
+            for child in children:
+                try:
+                    child.close()
+                except Exception:
+                    pass
+        except Exception:
+            pass
+
+        # 5. Close the OpenAI/httpx client
+        try:
+            client = getattr(self, "client", None)
+            if client is not None:
+                self._close_openai_client(client, reason="agent_close", shared=True)
+                self.client = None
+        except Exception:
+            pass
+
     def _hydrate_todo_store(self, history: List[Dict[str, Any]]) -> None:
         """
         Recover todo state from conversation history.