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>

lib/idea-code.ts

@@ -0,0 +1,8 @@
+// Pure helpers voor IDEA-codes. Geen DB-imports — daarom client-safe.
+// De DB-mutating nextIdeaCode staat in lib/idea-code-server.ts.
+
+const PAD = 3 // "IDEA-001". Bumps to 4 digits at counter 1000 organically.
+
+export function formatIdeaCode(n: number): string {
+  return `IDEA-${String(n).padStart(PAD, '0')}`
+}