feat(agent): Agent 实体（指令/工具/guardrail/handoff + 单步 RunStep，含多模态路由/调用记录）

internal/runtime/agent/handoff.go

@@ -0,0 +1,22 @@
+package agent
+
+// Handoff is a placeholder describing a transfer of control to another character
+// or subagent. It deliberately carries no orchestration: wiring the handover to
+// the agent-to-agent bus and subagent lifecycle belongs to the orchestration
+// layer (M3, DevDocs/design/04-运行时.md §2).
+type Handoff struct {
+	// Target is the identity (character or subagent) receiving control.
+	Target string
+	// Reason explains why the agent is handing off, for the orchestrator and
+	// audit logs.
+	Reason string
+}
+
+// NewHandoff builds a Handoff placeholder for the given target and reason.
+func NewHandoff(target, reason string) *Handoff {
+	return &Handoff{Target: target, Reason: reason}
+}
+
+// Handoff returns the agent's configured handoff placeholder, or nil when none
+// was set. It is read-only: orchestration interprets it without mutating it.
+func (a *Agent) Handoff() *Handoff { return a.handoff }