feat: AutoZero 通用 Agent 框架（从 AutoGo 五阶段拆分完成）

llm（三协议 Provider）/ session（JSONL 持久化）/ tools（Registry、围栏、
builtin、computer）/ cdp（browser）/ skills / mcp / agent（循环、两级压缩、
编排、审批门控）+ 底座 jsonl/textx/execx/diag/scratch/spill/imagecache。

产品装配代码（config 桥接、schedule 工具、memory/plan/retro/prompt）
留在 AutoGo，宿主接入方式见 MIGRATION.md。

llm/system_cache_test.go

@@ -0,0 +1,107 @@
+package llm
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestSystemCacheSplit：稳定 system 与动态 system 必须分开下发，且收尾回合
+// 只禁用工具调用、不撤掉工具定义——两者都是为了保住 prompt cache 前缀。
+func TestSystemCacheSplit(t *testing.T) {
+	tools := []ToolDef{{Name: "read", Description: "d", Parameters: map[string]any{"type": "object"}}}
+	opts := StreamOptions{SystemDynamic: "DYNAMIC", DisableTools: true}
+
+	t.Run("anthropic：动态段是第二个 system 块且不带 cache_control", func(t *testing.T) {
+		var body map[string]any
+		srv := captureBodyServer(t, &body,
+			"data: {\"type\":\"content_block_delta\",\"index\":0,\"delta\":{\"type\":\"text_delta\",\"text\":\"ok\"}}\n\n"+
+				"data: {\"type\":\"message_stop\"}\n\n")
+		defer srv.Close()
+		p := &AnthropicProvider{BaseURL: srv.URL, APIKey: "k", Model: "m", MaxTokens: 128, HTTP: srv.Client()}
+		if _, err := p.Stream(context.Background(), "STABLE",
+			[]Message{{Role: RoleUser, Content: "hi"}}, tools, opts, nil); err != nil {
+			t.Fatal(err)
+		}
+		blocks := body["system"].([]any)
+		if len(blocks) != 2 {
+			t.Fatalf("want 2 system blocks, got %d: %v", len(blocks), blocks)
+		}
+		first := blocks[0].(map[string]any)
+		second := blocks[1].(map[string]any)
+		if first["text"] != "STABLE" || first["cache_control"].(map[string]any)["type"] != "ephemeral" {
+			t.Fatalf("stable block must be cached: %v", first)
+		}
+		if second["text"] != "DYNAMIC" {
+			t.Fatalf("dynamic block text: %v", second)
+		}
+		if _, ok := second["cache_control"]; ok {
+			t.Fatalf("dynamic block must not carry cache_control: %v", second)
+		}
+		if body["tool_choice"].(map[string]any)["type"] != "none" {
+			t.Fatalf("tool_choice: %v", body["tool_choice"])
+		}
+		if len(body["tools"].([]any)) != 1 {
+			t.Fatalf("tools must stay in the request: %v", body["tools"])
+		}
+	})
+
+	t.Run("chat_completions：动态段是独立的第二条 system 消息", func(t *testing.T) {
+		var body map[string]any
+		srv := captureBodyServer(t, &body,
+			"data: {\"choices\":[{\"delta\":{\"content\":\"ok\"},\"finish_reason\":\"stop\"}]}\n\ndata: [DONE]\n\n")
+		defer srv.Close()
+		p := &OpenAIProvider{BaseURL: srv.URL, APIKey: "k", Model: "m", HTTP: srv.Client()}
+		if _, err := p.Stream(context.Background(), "STABLE",
+			[]Message{{Role: RoleUser, Content: "hi"}}, tools, opts, nil); err != nil {
+			t.Fatal(err)
+		}
+		msgs := body["messages"].([]any)
+		if len(msgs) != 2 {
+			t.Fatalf("want one system message + user, got %v", msgs)
+		}
+		if msgs[0].(map[string]any)["content"] != "STABLE\n\nDYNAMIC" {
+			t.Fatalf("system must keep the stable prefix and append the dynamic tail: %v", msgs[0])
+		}
+		if body["tool_choice"] != "none" {
+			t.Fatalf("tool_choice: %v", body["tool_choice"])
+		}
+		if len(body["tools"].([]any)) != 1 {
+			t.Fatalf("tools must stay in the request: %v", body["tools"])
+		}
+	})
+
+	t.Run("responses：动态段是独立的第二条 system 消息", func(t *testing.T) {
+		var body map[string]any
+		srv := captureBodyServer(t, &body,
+			"data: {\"type\":\"response.output_text.delta\",\"delta\":\"ok\"}\n\n"+
+				"data: {\"type\":\"response.completed\",\"response\":{\"output\":[],\"usage\":{\"input_tokens\":1,\"output_tokens\":1}}}\n\n")
+		defer srv.Close()
+		p := &ResponsesProvider{BaseURL: srv.URL, APIKey: "k", Model: "m", HTTP: srv.Client()}
+		if _, err := p.Stream(context.Background(), "STABLE",
+			[]Message{{Role: RoleUser, Content: "hi"}}, tools, opts, nil); err != nil {
+			t.Fatal(err)
+		}
+		input := body["input"].([]any)
+		if len(input) != 2 {
+			t.Fatalf("want one system message + user, got %v", input)
+		}
+		if input[0].(map[string]any)["content"] != "STABLE\n\nDYNAMIC" {
+			t.Fatalf("system must keep the stable prefix and append the dynamic tail: %v", input[0])
+		}
+		if body["tool_choice"] != "none" {
+			t.Fatalf("tool_choice: %v", body["tool_choice"])
+		}
+	})
+}
+
+func captureBodyServer(t *testing.T, body *map[string]any, sse string) *httptest.Server {
+	t.Helper()
+	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_ = json.NewDecoder(r.Body).Decode(body)
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = w.Write([]byte(sse))
+	}))
+}