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/request.go

@@ -0,0 +1,68 @@
+package agent
+
+import "git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/model"
+
+// defaultPurpose labels CallRecords when Options.Purpose is empty.
+const defaultPurpose = "agent"
+
+// buildRequest assembles a model.Request for one step: the agent's instructions
+// prepended as a leading system message, plus the conversation messages, the
+// configured model/temperature/max-tokens/purpose and the registered tool
+// schemas. When a router is injected it overrides the model from the request
+// contents, which is how vision requests are routed to the vision model.
+func (a *Agent) buildRequest(messages []model.Message) model.Request {
+	req := model.Request{
+		Model:       a.opts.Model,
+		Messages:    a.assembleMessages(messages),
+		Temperature: a.temperature(),
+		MaxTokens:   a.opts.MaxTokens,
+		Purpose:     a.purpose(),
+		Tools:       a.toolSchemas(),
+	}
+	if a.router != nil {
+		if routeModel, _ := a.router.RouteRequest(req); routeModel != "" {
+			req.Model = routeModel
+		}
+	}
+	return req
+}
+
+// assembleMessages returns messages with the agent's instructions prepended as a
+// leading system message when instructions are non-empty, otherwise the input
+// slice is returned unchanged.
+func (a *Agent) assembleMessages(messages []model.Message) []model.Message {
+	if a.instructions == "" {
+		return messages
+	}
+	out := make([]model.Message, 0, len(messages)+1)
+	out = append(out, model.TextMessage(model.RoleSystem, a.instructions))
+	out = append(out, messages...)
+	return out
+}
+
+// temperature returns the temperature pointer to place in a Request. Zero maps
+// to nil so the provider default applies.
+func (a *Agent) temperature() *float64 {
+	if a.opts.Temperature == 0 {
+		return nil
+	}
+	t := a.opts.Temperature
+	return &t
+}
+
+// purpose returns the CallRecord purpose label, defaulting to defaultPurpose.
+func (a *Agent) purpose() string {
+	if a.opts.Purpose == "" {
+		return defaultPurpose
+	}
+	return a.opts.Purpose
+}
+
+// toolSchemas returns the registered tool schemas advertised to the model, or
+// nil when no dispatcher is configured.
+func (a *Agent) toolSchemas() []model.ToolSpec {
+	if a.tools == nil {
+		return nil
+	}
+	return a.tools.ToolSchemas()
+}