lib: idea-code generator + plan_md yaml-frontmatter parser (M12 T-494)

- lib/idea-code.ts: pure formatIdeaCode helper (client-safe, no prisma)
- lib/idea-code-server.ts: atomic nextIdeaCode via Prisma row-lock,
  accepts optional TransactionClient for combining with idea.create
- lib/idea-plan-parser.ts: parsePlanMd extracts ---yaml---/body, runs
  yaml.parse + ideaPlanMdFrontmatterSchema, returns line-info on failure;
  CRLF-tolerant
- adds yaml@^2.8.4 dependency
- 8 unit tests (parser happy/missing/yaml-error/zod-error/empty-stories/CRLF;
  formatIdeaCode pad-3 + overflow)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

__tests__/lib/idea-code.test.ts

@@ -0,0 +1,21 @@
+import { describe, it, expect } from 'vitest'
+
+import { formatIdeaCode } from '@/lib/idea-code'
+
+describe('formatIdeaCode', () => {
+  it('pads to 3 digits', () => {
+    expect(formatIdeaCode(1)).toBe('IDEA-001')
+    expect(formatIdeaCode(42)).toBe('IDEA-042')
+    expect(formatIdeaCode(999)).toBe('IDEA-999')
+  })
+
+  it('does not truncate beyond pad-width', () => {
+    expect(formatIdeaCode(1000)).toBe('IDEA-1000')
+    expect(formatIdeaCode(99999)).toBe('IDEA-99999')
+  })
+})
+
+// Integration-style concurrency-test op nextIdeaCode is in
+// __tests__/integration/ tests die de echte DB raken (zie M12 verificatie-stap).
+// Hier alleen de pure formatter; de increment-logica leunt op Prisma's
+// row-lock in $transaction die we per-database vertrouwen.