refactor(mcp-go): 拆除 search.Hybrid 残骸（T4.F）

- search.Hybrid 空转(NewHybrid 返空、Query 返 nil TODO)、构造后存进 Gateway
  却从不被调用（真实 RAG 走 rag.Engine）→ 纯误导性死重量
- 删 internal/search 包 + gateway.go 字段/构造参数 + main.go 接线
- build/vet 干净；wiki_search 不受影响（本就用 g.rag）

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

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

@@ -21,13 +21,11 @@ import (
 	"github.com/sundynix/sundynix-mcp-go/internal/memory"
 	"github.com/sundynix/sundynix-mcp-go/internal/office"
 	"github.com/sundynix/sundynix-mcp-go/internal/rag"
-	"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
 	history *history.Store
 	rag     *rag.Engine
@@ -60,8 +58,8 @@ type toolDef struct {
 	handler   func(context.Context, *contract.ToolCall) *contract.ToolResult
 }
 
-func NewGateway(b *sharedbus.Bus, s *search.Hybrid, m *memory.Store, h *history.Store, r *rag.Engine, pgDSN string) *Gateway {
-	g := &Gateway{bus: b, search: s, memory: m, history: h, rag: r, pgDSN: pgDSN}
+func NewGateway(b *sharedbus.Bus, m *memory.Store, h *history.Store, r *rag.Engine, pgDSN string) *Gateway {
+	g := &Gateway{bus: b, memory: m, history: h, rag: r, pgDSN: pgDSN}
 	g.tools = g.buildRegistry()
 	return g
 }