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/migrations/20260228000000_ms22_agent_memory/migration.sql

@@ -0,0 +1,24 @@
+-- CreateTable
+CREATE TABLE "agent_memories" (
+    "id" UUID NOT NULL,
+    "workspace_id" UUID NOT NULL,
+    "agent_id" TEXT NOT NULL,
+    "key" TEXT NOT NULL,
+    "value" JSONB NOT NULL,
+    "created_at" TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMPTZ NOT NULL,
+
+    CONSTRAINT "agent_memories_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "agent_memories_workspace_id_agent_id_key_key" ON "agent_memories"("workspace_id", "agent_id", "key");
+
+-- CreateIndex
+CREATE INDEX "agent_memories_workspace_id_idx" ON "agent_memories"("workspace_id");
+
+-- CreateIndex
+CREATE INDEX "agent_memories_agent_id_idx" ON "agent_memories"("agent_id");
+
+-- AddForeignKey
+ALTER TABLE "agent_memories" ADD CONSTRAINT "agent_memories_workspace_id_fkey" FOREIGN KEY ("workspace_id") REFERENCES "workspaces"("id") ON DELETE CASCADE ON UPDATE CASCADE;