Add handoff agent-workflow builder (implements #520)

[PratikDhanave]

Implements the handoff orchestration gap from #520. Opening as a draft to align on the API/mechanism before finalizing — happy to adjust to the team's steer (the design questions in #520 are resolved here with sensible defaults, noted below).

What

A first-class handoff builder alongside the existing sequential / concurrent / group-chat builders:

wf, err := agentworkflow.NewHandoffWorkflowBuilder(triage, billing, refunds).
    WithHandoff(triage, billing, refunds). // triage may hand off to billing or refunds
    WithHandoff(billing, triage).          // billing may hand back to triage
    WithOutputFrom(triage, billing, refunds).
    Build()

A handoff workflow is a group chat whose next speaker is chosen by the current agent: each agent is given a handoff_to_<target> tool for every target it may hand off to; calling that tool routes the shared conversation to the target. The first agent is the entry agent; a turn with no handoff call ends the workflow.

How (reuse, not a new engine)

• NewHandoffWorkflowBuilder injects per-agent handoff tools and supplies a handoff-flavored GroupChatManager whose SelectNextAgent routes on the most recent handoff tool call in the previous speaker's turn (a cursor tracks per-turn scanning and is checkpoint-persisted, mirroring the round-robin manager's NextIndex).
• Rides the existing groupChatHostExecutor, edges, output designations, and prefixingWorkflowContext checkpoint plumbing.
• One additive change to agentworkflow.Config: a RunOptions []agent.Option field so the builder can inject the handoff tools into each hosted agent's run (hosting.go). No existing signatures change.

Design decisions (the open questions from #520)

1. Tools, not edges — .NET-faithful auto-injected handoff_to_<target> function tools. Because they carry a handler they auto-execute, so the (resolved) call flows into history for the manager to read — no InterceptUnterminatedFunctionCalls needed.
2. Reuse the group-chat manager — handoff is a GroupChatManager variant; no duplicate engine.
3. Termination — a turn with no handoff call ends the workflow (plus the host's existing 40-turn safety cap).
4. Return-to-previous — supported by declaring reverse handoffs (WithHandoff(billing, triage)).
5. Scope — handoff only; Magentic tracked separately.

Tests

handoff_test.go (white-box, matching this package's convention) covers: routing on a handoff call, hand-back (triage→billing→triage), no-handoff termination, builder validation (nil/non-participant agents & targets), and handoff-manager checkpoint restore. All green under -race -shuffle=on; full suite passes. Updates docs/dotnet-go-sdk-feature-comparison.md.

Open for steer

Naming (WithHandoff vs edge-style), whether handoff instructions should be auto-added to agent system prompts (.NET does), and whether to add an examples/03-workflows sample once the API is agreed — I'll follow up per the team's preference.