feat(PBI-49): add debugProps helper + Vitest test

Adds lib/debug.ts with debugProps(id, component, file) that returns
data-debug-id and data-debug-label attrs in dev mode, empty object in
production. Adds __tests__/lib/debug.test.ts covering both modes.

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

__tests__/lib/debug.test.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect, vi } from 'vitest'
+
+import { debugProps } from '@/lib/debug'
+
+describe('debugProps', () => {
+  it('returns both attrs with correct label in dev mode', () => {
+    const result = debugProps('sprint-board', 'SprintBoard', 'components/sprint/sprint-board.tsx')
+    expect(result).toEqual({
+      'data-debug-id': 'sprint-board',
+      'data-debug-label': 'SprintBoard — components/sprint/sprint-board.tsx',
+    })
+  })
+
+  it('returns empty object in production mode', () => {
+    const original = process.env.NODE_ENV
+    try {
+      vi.stubEnv('NODE_ENV', 'production')
+      const result = debugProps('sprint-board', 'SprintBoard', 'components/sprint/sprint-board.tsx')
+      expect(result).toEqual({})
+    } finally {
+      vi.stubEnv('NODE_ENV', original ?? 'test')
+    }
+  })
+})

lib/debug.ts

@@ -0,0 +1,16 @@
+export type DebugProps = {
+  'data-debug-id': string
+  'data-debug-label': string
+}
+
+export function debugProps(
+  id: string,
+  component: string,
+  file: string
+): DebugProps | Record<string, never> {
+  if (process.env.NODE_ENV === 'production') return {}
+  return {
+    'data-debug-id': id,
+    'data-debug-label': `${component} — ${file}`,
+  }
+}