feat(ST-1112): add updateTaskStatusWithStoryPromotion helper

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

lib/tasks-status-update.ts

@@ -0,0 +1,72 @@
+import type { Prisma, TaskStatus } from '@prisma/client'
+import { prisma } from '@/lib/prisma'
+
+export type StoryStatusChange = 'promoted' | 'demoted' | null
+
+export interface UpdateTaskStatusResult {
+  task: {
+    id: string
+    title: string
+    status: TaskStatus
+    story_id: string
+    implementation_plan: string | null
+  }
+  storyStatusChange: StoryStatusChange
+  storyId: string
+}
+
+// Update task.status atomically and auto-promote/demote the parent story:
+//   - All sibling tasks DONE → story.status = DONE
+//   - Story was DONE and a task moves out of DONE → story.status = IN_SPRINT
+// Demote target is IN_SPRINT (not OPEN): OPEN means "back in product backlog",
+// which is a sprint-management action, not a status side-effect.
+export async function updateTaskStatusWithStoryPromotion(
+  taskId: string,
+  newStatus: TaskStatus,
+  client?: Prisma.TransactionClient,
+): Promise<UpdateTaskStatusResult> {
+  const run = async (tx: Prisma.TransactionClient): Promise<UpdateTaskStatusResult> => {
+    const task = await tx.task.update({
+      where: { id: taskId },
+      data: { status: newStatus },
+      select: {
+        id: true,
+        title: true,
+        status: true,
+        story_id: true,
+        implementation_plan: true,
+      },
+    })
+
+    const siblings = await tx.task.findMany({
+      where: { story_id: task.story_id },
+      select: { status: true },
+    })
+    const allDone = siblings.every((s) => s.status === 'DONE')
+
+    const story = await tx.story.findUniqueOrThrow({
+      where: { id: task.story_id },
+      select: { status: true },
+    })
+
+    let storyStatusChange: StoryStatusChange = null
+    if (newStatus === 'DONE' && allDone && story.status !== 'DONE') {
+      await tx.story.update({
+        where: { id: task.story_id },
+        data: { status: 'DONE' },
+      })
+      storyStatusChange = 'promoted'
+    } else if (newStatus !== 'DONE' && story.status === 'DONE') {
+      await tx.story.update({
+        where: { id: task.story_id },
+        data: { status: 'IN_SPRINT' },
+      })
+      storyStatusChange = 'demoted'
+    }
+
+    return { task, storyStatusChange, storyId: task.story_id }
+  }
+
+  if (client) return run(client)
+  return prisma.$transaction(run)
+}