Add handoff agent-workflow builder (implements #520)

docs/dotnet-go-sdk-feature-comparison.md

@@ -72,7 +72,7 @@ Within overlapping features, the main misalignments are API shape and ecosystem
 | RAG | Basic text RAG, custom vector store RAG, custom data source RAG, Foundry service RAG, Neo4j graph RAG samples. | No RAG package or sample found. | .NET only | Go has data/file/vector content types but no RAG workflow package or samples. |
 | Purview | `Microsoft.Agents.AI.Purview` models and end-to-end sample. | No equivalent package. | .NET only | No Go governance/Purview integration. |
 | Cosmos DB storage | Cosmos chat history provider and workflow checkpoint store. | No built-in Cosmos package. | .NET only | Go has public in-memory and JSON/file workflow checkpoint stores plus a custom store interface, but no Cosmos DB provider. |
-| Agent workflow builders | Sequential, concurrent, handoff, group chat builders. | `agentworkflow.NewSequentialWorkflowBuilder`, `agentworkflow.NewConcurrentWorkflowBuilder`, `agentworkflow.NewGroupChatWorkflowBuilder`; manual builder plus `AddChain`, `AddSwitch`, direct/fan-out/fan-in edges; workflow-as-agent, group chat, and agents-in-workflows examples. | Partial | Go now has first-class sequential, concurrent, and group chat builders with explicit output designation support. Handoff and Magentic builders are not yet implemented. |
+| Agent workflow builders | Sequential, concurrent, handoff, group chat builders. | `agentworkflow.NewSequentialWorkflowBuilder`, `agentworkflow.NewConcurrentWorkflowBuilder`, `agentworkflow.NewGroupChatWorkflowBuilder`, `agentworkflow.NewHandoffWorkflowBuilder`; manual builder plus `AddChain`, `AddSwitch`, direct/fan-out/fan-in edges; workflow-as-agent, group chat, and agents-in-workflows examples. | Partial | Go now has first-class sequential, concurrent, group chat, and handoff builders with explicit output designation support. The Magentic builder is not yet implemented. |
 | Workflow graph builder | `WorkflowBuilder`, direct edges, fan-out, fan-in barrier, labels, conditions, switch/case samples. | `workflow.Builder`, `AddEdge`, `AddDirectEdge`, `AddFanOutEdge`, `AddFanInBarrierEdge`, `WithEdgeLabel`, `WithEdgeAssigner`, `AddSwitch`. | Aligned | .NET has more overloads/extension methods; Go uses simpler methods and option functions. |
 | Workflow executor model | Generic `Executor<TInput>` and `Executor<TInput,TOutput>`, function executors, aggregating executor, protocol builder. | `Executor`, `NewExecutor`, `Executor.Bind`, `Executor.Extend`, `RouteBuilder`, `StatefulExecutorCache`. | Partial | .NET has more overloads and an explicit `AggregatingExecutor`; Go mirrors .NET's executor-level cross-run declaration and binding-level concurrent-run gate, while route configuration and lifecycle hooks live on `Executor`. |
 | Workflow protocol description | Accepts/yields/sends/catch-all protocol descriptor and chat protocol helpers. | `ProtocolDescriptor` exposes accepted, yielded, and sent types plus catch-all acceptance; `messageworkflow.Configure` contributes chat-message protocol metadata. | Aligned | Go now exposes the same protocol shape while keeping chat helpers in the Go-specific message workflow adapter. |
@@ -84,7 +84,7 @@ Within overlapping features, the main misalignments are API shape and ecosystem
 | Agent in workflow | `AIAgentBinding`, `AIAgentHostOptions`, response/update events, role reassignment, message forwarding, intercept user-input/function-call requests. | `workflow/agentworkflow.New` with `Config`: update/response events, message forwarding toggle (`DisableForwardIncomingMessages`), role reassignment toggle (`DisableReassignOtherAgentsAsUsers`), `InterceptUserInputRequests`, `InterceptUnterminatedFunctionCalls`. | Aligned | API shape differs: .NET uses positive-boolean defaults (`ForwardIncomingMessages = true`, `ReassignOtherAgentsAsUsers = true`); Go uses opt-out booleans (`DisableForwardIncomingMessages`, `DisableReassignOtherAgentsAsUsers`). Feature coverage is equivalent. |
 | Workflow as agent | Workflow host agent / `AsAIAgent`, sample. | `agentworkflow.NewAgent`. | Aligned | Go chooses in-process environment based on concurrency; .NET is integrated with `AIAgent` extensions. |
 | Subworkflows | `ConfigureSubWorkflow`, `BindAsExecutor`, subworkflow sample. | `inproc.BindSubworkflowAsExecutor` plus in-process subworkflow execution. | Aligned | Go exposes subworkflow binding from the in-process execution package. |
-| Handoff orchestration | Handoff workflow builder with handoff instructions, tool-call filtering, return-to-previous, response/update events. | No first-class handoff builder. | .NET only | Could be modeled manually with tools/workflows, but no SDK feature. |
+| Handoff orchestration | Handoff workflow builder with handoff instructions, tool-call filtering, return-to-previous, response/update events. | `agentworkflow.NewHandoffWorkflowBuilder` with `WithHandoff` handoff edges; per-agent handoff tools route the shared conversation to the chosen target, with return-to-previous supported by declaring reverse handoffs. | Aligned | Go now has a first-class handoff builder; the current turn's agent hands off by calling an injected `handoff_to_<target>` tool, reusing the group chat host and manager plumbing. |
 | Group chat orchestration | Group chat manager and group chat workflow builder. | `agentworkflow.GroupChatManager`, `agentworkflow.NewRoundRobinGroupChatManager`, `agentworkflow.NewGroupChatWorkflowBuilder`. | Aligned | API shape differs, but Go now has a managed group chat abstraction with manager callbacks and workflow hosting. |
 | Declarative agents | YAML prompt/declarative agent packages, factories, PowerFx helpers. | No equivalent package. | .NET only | Go skills are prompt-like, but not declarative agents. |
 | Declarative workflows | Declarative workflow packages, Foundry/MCP declarative integrations, samples for confirm input, HTTP, code, MCP, function tools, marketing, student/teacher, etc. | No equivalent package. | .NET only | Go workflows are code-first. |