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>

package.json

@@ -55,6 +55,7 @@
     "sonner": "^1.7.4",
     "tailwind-merge": "^3.5.0",
     "tw-animate-css": "^1.4.0",
+    "yaml": "^2.8.4",
     "zod": "^3.25.76",
     "zustand": "^5.0.12"
   },