feat(#29): implement cron job configuration

- Add CronSchedule model to Prisma schema
- Implement CronService with CRUD operations
- Add REST API endpoints for cron management
- Create MoltBot plugin skill definition (SKILL.md)
- TDD: 9 passing tests for CronService

apps/api/prisma/schema.prisma

@@ -187,6 +187,7 @@ model Workspace {
   userLayouts      UserLayout[]
   knowledgeEntries KnowledgeEntry[]
   knowledgeTags    KnowledgeTag[]
+  cronSchedules    CronSchedule[]
 
   @@index([ownerId])
   @@map("workspaces")
@@ -808,3 +809,31 @@ model KnowledgeEmbedding {
   @@index([entryId])
   @@map("knowledge_embeddings")
 }
+
+// ============================================
+// CRON JOBS
+// ============================================
+
+model CronSchedule {
+  id          String   @id @default(uuid()) @db.Uuid
+  workspaceId String   @map("workspace_id") @db.Uuid
+  workspace   Workspace @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
+
+  // Cron configuration
+  expression String  // Standard cron: "0 9 * * *" = 9am daily
+  command    String  // MoltBot command to trigger
+
+  // State
+  enabled  Boolean @default(true)
+  lastRun  DateTime? @map("last_run") @db.Timestamptz
+  nextRun  DateTime? @map("next_run") @db.Timestamptz
+
+  // Audit
+  createdAt DateTime @default(now()) @map("created_at") @db.Timestamptz
+  updatedAt DateTime @updatedAt @map("updated_at") @db.Timestamptz
+
+  @@index([workspaceId])
+  @@index([workspaceId, enabled])
+  @@index([nextRun])
+  @@map("cron_schedules")
+}