feat: 完成 C 群組 — 記憶子系統

- WorkingMemoryService：session 對話上下文緩衝，token 溢位時優先裁切最舊的低情緒段落
- JobQueue 抽象層（now/schedule(at)/every(cron)）與 InProcessJobQueue 實作（node-cron），
  R-6 會換成 BullMQ 但呼叫端介面不變
- MemoryConsolidationService：睡眠固化，只有高情緒或重複提及的內容才寫入 EpisodicMemory
- ForgettingSweepService：低情緒且長期未提取的記憶逐次降權、weight 過低後刪除，
  已掛上每小時一次的 JobQueue.every 排程
- KeywordMemoryRetriever：關鍵字＋時間近因＋情緒權重排序，提取命中即更新提取次數／時間
- apps/api 新增 /memory/* 端點作為引擎驗證介面（H 群組會決定併入正式對話管線後的去留）
- scripts/smoke/C.mjs：端到端驗證溢位裁切、只寫工作記憶、固化篩選、檢索排序、
  提取即改寫、schedule(at) 準時觸發、遺忘衰減與刪除

npm run restart && npm run smoke -- C 皆通過（C-V），A/B 群組冒煙測試無回歸。

apps/api/src/memory/memory-retriever.service.ts

@@ -0,0 +1,69 @@
+import { Injectable } from "@nestjs/common";
+import type { EpisodicMemory } from "@kokorone/shared";
+import { PrismaService } from "../prisma/prisma.service.js";
+
+export interface MemoryRetriever {
+  retrieve(characterId: string, query: string, limit?: number): Promise<EpisodicMemory[]>;
+}
+
+const WEIGHTS = {
+  keyword: 0.5,
+  recency: 0.3,
+  emotion: 0.2,
+};
+
+const DEFAULT_LIMIT = 5;
+
+function keywordRelevance(content: string, keywords: string[]): number {
+  if (keywords.length === 0) {
+    return 0;
+  }
+  const hits = keywords.filter((keyword) => content.includes(keyword)).length;
+  return hits / keywords.length;
+}
+
+// C-6 記憶檢索器（關鍵字版本）：排序權重＝關鍵字相關度＋時間近因＋情緒權重。
+// R-2 會加上 pgvector 語意檢索實作，取代這裡的關鍵字比對。
+@Injectable()
+export class KeywordMemoryRetriever implements MemoryRetriever {
+  constructor(private readonly prisma: PrismaService) {}
+
+  async retrieve(characterId: string, query: string, limit = DEFAULT_LIMIT): Promise<EpisodicMemory[]> {
+    const rows = await this.prisma.client.episodicMemory.findMany({ where: { characterId } });
+    const now = Date.now();
+    const keywords = query.split(/\s+/).filter(Boolean);
+
+    const scored = rows.map((row) => {
+      const keywordScore = keywordRelevance(row.content, keywords);
+      const daysSince = (now - row.occurredAt.getTime()) / (1000 * 60 * 60 * 24);
+      const recencyScore = 1 / (1 + Math.max(0, daysSince));
+      const emotionScore = row.emotionIntensity;
+      const score =
+        keywordScore * WEIGHTS.keyword + recencyScore * WEIGHTS.recency + emotionScore * WEIGHTS.emotion;
+      return { row, score };
+    });
+
+    scored.sort((a, b) => b.score - a.score);
+    const top = scored.slice(0, limit).map((s) => s.row);
+
+    if (top.length > 0) {
+      await this.prisma.client.episodicMemory.updateMany({
+        where: { id: { in: top.map((row) => row.id) } },
+        data: { lastRetrievedAt: new Date(now), retrievalCount: { increment: 1 } },
+      });
+    }
+
+    return top.map((row) => ({
+      id: row.id,
+      characterId: row.characterId,
+      content: row.content,
+      occurredAt: row.occurredAt.toISOString(),
+      emotionTag: row.emotionTag,
+      emotionIntensity: row.emotionIntensity,
+      retrievalCount: row.retrievalCount + 1,
+      lastRetrievedAt: new Date(now).toISOString(),
+      source: row.source,
+      weight: row.weight,
+    }));
+  }
+}