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>

apps/gateway/package.json

@@ -18,6 +18,7 @@
     "@mosaic/brain": "workspace:^",
     "@mosaic/coord": "workspace:^",
     "@mosaic/db": "workspace:^",
+    "@mosaic/memory": "workspace:^",
     "@mosaic/types": "workspace:^",
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",