feat(memory): M4 记忆完善（三类作用域访问规则+项目索引分层+LLM异步提取+衰减溯源+向量接入，兼容既有57用例）

internal/runtime/memory/extract.go

@@ -2,17 +2,27 @@ package memory
 
 import (
 	"context"
+	"errors"
 	"strings"
 
 	"git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/model"
 )
 
+var (
+	// ErrNilAdapter is returned when an LLMExtractor is built or used without a
+	// model.Adapter to drive extraction.
+	ErrNilAdapter = errors.New("memory: nil model adapter")
+)
+
 // Extractor derives candidate memory items from a conversation (design §1).
 //
-// The production path is an LLM-driven asynchronous extractor that denoises and
-// deduplicates each turn; that is deferred to milestone M4, and its integration
-// point is this interface. The M2 implementation is the rule-based
-// ManualExtractor, which only records what the user explicitly asks to remember.
+// Two implementations exist:
+//
+//   - LLMExtractor: asks a model.Adapter to denoise and deduplicate each turn,
+//     running synchronously (Extract) or on a background goroutine
+//     (ExtractAsync).
+//   - ManualExtractor: deterministic, rule-based — it only records what the user
+//     explicitly asks to remember ("记住 X").
 type Extractor interface {
 	// Extract returns memory items worth persisting from conversation. ctx
 	// controls cancellation; a canceled context returns an error.