feat: 短期多轮历史接入 Eino 图 MessagesPlaceholder (⑨)

会话历史(Redis,易失,与长期画像分开)经 MCP 工具进出 Eino 图：
recall 召回历史填 MessagesPlaceholder，写回把本轮 user/assistant 落历史。

- mcp-go: internal/history(go-redis, sundynix:history:<session>, LPUSH+LTRIM 保留近20条,
  24h TTL) + 工具 history_get(返回JSON turns)/history_append; main 开 Redis(降级)
- dispatcher Eino: 模板加 MessagesPlaceholder('history'); recall 调 history_get→转 schema.Message;
  Handle 累积 answer; memorize 异步 history_append(user+assistant)
- shared: contract.MetaSessionID; gateway: SubmitTask 注入 Meta[session_id](X-Session-ID 头,缺省 default)
- demo.sh: 同会话两轮提交，验证第2轮召回第1轮历史
- 验证: 4 模块 build✓ + 3 e2e PASS; live 跑通——轮1=0轮历史→落库, 轮2 history_get 命中→注入

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-mcp-go/internal/mcp/gateway.go

@@ -3,25 +3,28 @@ package mcp
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"log"
 
 	sharedbus "github.com/sundynix/sundynix-shared/bus"
 	"github.com/sundynix/sundynix-shared/contract"
 
+	"github.com/sundynix/sundynix-mcp-go/internal/history"
 	"github.com/sundynix/sundynix-mcp-go/internal/memory"
 	"github.com/sundynix/sundynix-mcp-go/internal/search"
 )
 
 // Gateway 暴露 MCP 协议端点，经共享 bus 订阅 sundynix.tools.go.* 响应调用。
 type Gateway struct {
-	bus    *sharedbus.Bus
-	search *search.Hybrid
-	memory *memory.Store
+	bus     *sharedbus.Bus
+	search  *search.Hybrid
+	memory  *memory.Store
+	history *history.Store
 }
 
-func NewGateway(b *sharedbus.Bus, s *search.Hybrid, m *memory.Store) *Gateway {
-	return &Gateway{bus: b, search: s, memory: m}
+func NewGateway(b *sharedbus.Bus, s *search.Hybrid, m *memory.Store, h *history.Store) *Gateway {
+	return &Gateway{bus: b, search: s, memory: m, history: h}
 }
 
 // Serve 以队列组通配订阅 sundynix.tools.go.>，按工具名分发并阻塞。
@@ -31,7 +34,7 @@ func (g *Gateway) Serve(ctx context.Context) error {
 		return err
 	}
 	defer func() { _ = unsub() }()
-	log.Printf("[mcp_go] tools ready on %s (queue=%s): wiki_search, memory_get, memory_upsert, echo",
+	log.Printf("[mcp_go] tools ready on %s (queue=%s): wiki_search, memory_get, memory_upsert, history_get, history_append, echo",
 		contract.SubjectToolsGoAll, contract.QueueToolsGo)
 	<-ctx.Done()
 	return ctx.Err()
@@ -47,6 +50,10 @@ func (g *Gateway) dispatch(ctx context.Context, call *contract.ToolCall) *contra
 		return g.memoryGet(ctx, call)
 	case "memory_upsert":
 		return g.memoryUpsert(ctx, call)
+	case "history_get":
+		return g.historyGet(ctx, call)
+	case "history_append":
+		return g.historyAppend(ctx, call)
 	case "echo":
 		return &contract.ToolResult{OK: true, Content: fmt.Sprint(call.Args["text"])}
 	default:
@@ -64,6 +71,31 @@ func (g *Gateway) memoryGet(ctx context.Context, call *contract.ToolCall) *contr
 	return &contract.ToolResult{OK: true, Content: profile}
 }
 
+// historyGet 召回某会话最近多轮历史，Content 为 JSON 数组 [{role,content},...]（正序）。
+func (g *Gateway) historyGet(ctx context.Context, call *contract.ToolCall) *contract.ToolResult {
+	session, _ := call.Args["session_id"].(string)
+	turns, err := g.history.Get(ctx, session)
+	if err != nil {
+		return &contract.ToolResult{OK: false, Error: "history_get: " + err.Error()}
+	}
+	data, _ := json.Marshal(turns)
+	return &contract.ToolResult{OK: true, Content: string(data)}
+}
+
+// historyAppend 追加一条会话消息（session_id + role + content）。
+func (g *Gateway) historyAppend(ctx context.Context, call *contract.ToolCall) *contract.ToolResult {
+	session, _ := call.Args["session_id"].(string)
+	role, _ := call.Args["role"].(string)
+	content, _ := call.Args["content"].(string)
+	if session == "" || role == "" {
+		return &contract.ToolResult{OK: false, Error: "history_append: session_id 和 role 必填"}
+	}
+	if err := g.history.Append(ctx, session, role, content); err != nil {
+		return &contract.ToolResult{OK: false, Error: "history_append: " + err.Error()}
+	}
+	return &contract.ToolResult{OK: true}
+}
+
 // memoryUpsert 写入/更新一条画像偏好（user_id + key + value）。
 func (g *Gateway) memoryUpsert(ctx context.Context, call *contract.ToolCall) *contract.ToolResult {
 	uid, _ := call.Args["user_id"].(string)