feat(P4-001): @mosaic/memory — preference + insight stores

Add memory tables to DB schema (preferences, insights with pgvector
embedding column, agent_logs, skills, summarization_jobs). Implement
PreferencesRepo (CRUD + upsert) and InsightsRepo (CRUD + semantic
search + relevance decay). Define VectorStore and EmbeddingProvider
interfaces for future provider abstraction. Wire MemoryModule into
gateway with REST endpoints at /api/memory/*.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/memory/src/memory.ts

@@ -0,0 +1,15 @@
+import type { Db } from '@mosaic/db';
+import { createPreferencesRepo, type PreferencesRepo } from './preferences.js';
+import { createInsightsRepo, type InsightsRepo } from './insights.js';
+
+export interface Memory {
+  preferences: PreferencesRepo;
+  insights: InsightsRepo;
+}
+
+export function createMemory(db: Db): Memory {
+  return {
+    preferences: createPreferencesRepo(db),
+    insights: createInsightsRepo(db),
+  };
+}