docs(add): add middleware execution flow diagram to custom.mdx

src/oss/langchain/middleware/custom.mdx

@@ -549,6 +549,20 @@ __Agent loop ends__
 - `after_*` hooks: Last to first (reverse)
 - `wrap_*` hooks: Nested (first middleware wraps all others)
 
+### Detailed execution flow
+
+The following diagram illustrates the complete agent execution lifecycle with all middleware hook points:
+
+<img
+    src="/oss/images/agent-middleware-execution-flow.svg"
+    alt="Agent middleware execution flow diagram showing the lifecycle from START through before_agent, before_model, model execution, after_model, tools execution, and after_agent hooks with their invocation order and relationships"
+    className="rounded-lg block mx-auto"
+/>
+
+<Note>
+Diagram is based on the [langchain_v1/langchain/agents/factory.py](https://github.com/langchain-ai/langchain/blob/81c4f21b5297b31b2cd82ccefdd134aa0e59efa0/libs/langchain_v1/langchain/agents/factory.py).
+</Note>
+
 ## Agent jumps
 
 To exit early from middleware, return a dictionary with `jump_to`: