docs: document creating and using a custom managed agent

Add a "Create and use a custom managed agent" section to the ManagedAgent guide
showing the control-plane create/delete pattern via `ManagedAgent.api_client`,
and complete the guide's Related samples list.

Co-authored-by: Haran Rajkumar <haranrk@google.com>
PiperOrigin-RevId: 954766292

Author: haranrk

docs/guides/agents/managed_agent/index.md

@@ -97,6 +97,37 @@ root_agent = LlmAgent(
 )
 ```
 
+## Create and use a custom managed agent
+
+`ManagedAgent` connects to an existing managed agent by `agent_id`. You can
+shape that agent's behavior inline, with no resource creation: set
+[`instruction`](#system-instruction) for a persona and pass server-side `tools`
+such as `google_search` (see [Get started](#get-started)).
+
+Create a **custom managed-agent resource** when you instead want a persistent,
+named agent whose persona and server-side tools are baked into the resource and
+reusable by id across apps and sessions. Create it through the control plane,
+then point `ManagedAgent` at its id. The genai client `ManagedAgent` already
+holds (`managed_search_agent.api_client`) exposes both planes: interactions
+(data plane) and `agents.create` / `agents.delete` (control plane), so you can
+create with the same client:
+
+```python
+created = managed_search_agent.api_client.agents.create(
+    id='adk-custom-search-agent',
+    base_agent='antigravity-preview-05-2026',
+    system_instruction='You are a concise research assistant. ...',
+    tools=[{'type': 'google_search'}],
+)
+# created.id is the agent id ManagedAgent(agent_id=...) connects to.
+```
+
+Creating a custom agent requires the GEAP/Vertex backend (`global` location);
+its `system_instruction` and tools are fixed at create time, and creation is
+asynchronous (the agent takes a short while to become ready). See the
+[custom_agent sample](../../../../contributing/samples/managed_agent/custom_agent)
+for a runnable example with `--create` / `--delete` flags.
+
 ## How it works
 
 The `ManagedAgent` implements the `BaseAgent` contract but bypasses standard
@@ -177,3 +208,6 @@ root_agent = ManagedAgent(
 *   [Managed Agent Basic](../../../../contributing/samples/managed_agent/basic)
 *   [Managed Agent Code Execution](../../../../contributing/samples/managed_agent/code_execution)
 *   [Managed Agent System Instruction](../../../../contributing/samples/managed_agent/system_instruction)
+*   [Managed Agent Remote MCP](../../../../contributing/samples/managed_agent/remote_mcp)
+*   [Managed Agent Single-Turn Orchestration](../../../../contributing/samples/managed_agent/single_turn)
+*   [Managed Agent Create and Use a Custom Agent](../../../../contributing/samples/managed_agent/custom_agent)