feat(api): add agent memory module (MS22-DB-002, MS22-API-002)

- Add AgentMemory model to schema.prisma with workspaceId/agentId/key/value
  and @@unique([workspaceId, agentId, key]) constraint
- Add Workspace.agentMemories relation
- Add migration 20260228000000_ms22_agent_memory
- Add AgentMemoryModule with service, controller, DTOs, and unit tests
- Endpoints: PUT/GET/GET/:key/DELETE on /api/agents/:agentId/memory/:key
- Register AgentMemoryModule in app.module.ts
- 10 unit tests passing (service + controller)

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

apps/api/prisma/schema.prisma

@@ -299,6 +299,7 @@ model Workspace {
   agentSessions                AgentSession[]
   agentTasks                   AgentTask[]
   findings                     Finding[]
+  agentMemories                AgentMemory[]
   userLayouts                  UserLayout[]
   knowledgeEntries             KnowledgeEntry[]
   knowledgeTags                KnowledgeTag[]
@@ -764,6 +765,23 @@ model AgentSession {
   @@map("agent_sessions")
 }
 
+model AgentMemory {
+  id          String   @id @default(uuid()) @db.Uuid
+  workspaceId String   @map("workspace_id") @db.Uuid
+  agentId     String   @map("agent_id")
+  key         String
+  value       Json
+  createdAt   DateTime @default(now()) @map("created_at") @db.Timestamptz
+  updatedAt   DateTime @updatedAt @map("updated_at") @db.Timestamptz
+
+  workspace Workspace @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
+
+  @@unique([workspaceId, agentId, key])
+  @@index([workspaceId])
+  @@index([agentId])
+  @@map("agent_memories")
+}
+
 model WidgetDefinition {
   id String @id @default(uuid()) @db.Uuid