fix(judge): parse balanced JSON responses

[mnkj0021]

Fixes #167

Summary

• replace regex-based bundled judge JSON extraction with a brace-balanced scanner
• ignore braces that occur inside JSON strings and handle escaped characters
• parse JSON independently of markdown code fences or trailing prose
• preserve the existing parse-failure behavior for genuinely truncated/incomplete JSON
• add regression coverage for nested JSON, braces in feedback strings, and truncated responses

Validation

• corepack pnpm --filter judge build — passed on a fresh recheck
• corepack pnpm exec vitest run apps/judge/src/judge-strategies.test.ts — 69/69 passed
• git diff --check — passed
• branch is current with upstream main (0 commits behind)

GitHub's upstream CI workflows are still in action_required and need a Scope maintainer to approve the fork workflow run. The Microsoft CLA check passes.