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>

pnpm-lock.yaml

@@ -59,6 +59,9 @@ importers:
       '@mosaic/db':
         specifier: workspace:^
         version: link:../../packages/db
+      '@mosaic/memory':
+        specifier: workspace:^
+        version: link:../../packages/memory
       '@mosaic/types':
         specifier: workspace:^
         version: link:../../packages/types
@@ -331,9 +334,15 @@ importers:
 
   packages/memory:
     dependencies:
+      '@mosaic/db':
+        specifier: workspace:*
+        version: link:../db
       '@mosaic/types':
         specifier: workspace:*
         version: link:../types
+      drizzle-orm:
+        specifier: ^0.45.1
+        version: 0.45.1(@opentelemetry/api@1.9.0)(@types/pg@8.15.6)(kysely@0.28.11)(postgres@3.4.8)
     devDependencies:
       typescript:
         specifier: ^5.8.0