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/src/agent-memory/dto/index.ts

@@ -0,0 +1 @@
+export * from "./upsert-agent-memory.dto";

apps/api/src/agent-memory/dto/upsert-agent-memory.dto.ts

@@ -0,0 +1,10 @@
+import { IsNotEmpty } from "class-validator";
+
+/**
+ * DTO for upserting an agent memory entry.
+ * The value accepts any JSON-serializable data.
+ */
+export class UpsertAgentMemoryDto {
+  @IsNotEmpty({ message: "value must not be empty" })
+  value!: unknown;
+}