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-lock.json

@@ -42,6 +42,7 @@
         "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"
       },
@@ -21518,10 +21519,9 @@
       "license": "ISC"
     },
     "node_modules/yaml": {
-      "version": "2.8.3",
-      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.8.3.tgz",
-      "integrity": "sha512-AvbaCLOO2Otw/lW5bmh9d/WEdcDFdQp2Z2ZUH3pX9U2ihyUY0nvLv7J6TrWowklRGPYbB/IuIMfYgxaCPg5Bpg==",
-      "dev": true,
+      "version": "2.8.4",
+      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.8.4.tgz",
+      "integrity": "sha512-ml/JPOj9fOQK8RNnWojA67GbZ0ApXAUlN2UQclwv2eVgTgn7O9gg9o7paZWKMp4g0H3nTLtS9LVzhkpOFIKzog==",
       "license": "ISC",
       "bin": {
         "yaml": "bin.mjs"