refactor(review): extract shared review contracts

[Terry-Mao]

Summary

• 抽取共享 review contract 和 JSON schema，统一 PR/spec/security review 的输出与证据规则
• 调整 review-pr/review-spec 为主入口，-repo skills 仅作为 companion guidance
• 将本地 review 快照和输出迁移到系统临时目录，并更新校验、文档和测试

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/tests
• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/aicodingflow-tests
• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/aicodingflow-tests -p 'test_validate_local_review_result.py'
• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/aicodingflow-tests -p 'test_prepare_local_review_inputs.py'
• git diff --check

Base Sync

• Fetched origin/main before opening this PR; current branch is not behind origin/main.

[github-actions]

Found: 0 critical, 1 important, 0 suggestions. The shared review contract refactor looks coherent overall, but the .gitignore change removes ignore coverage for non-review workflow handoff files that are still produced at the repository root.

[github-actions]

Found: 0 critical, 1 important, 0 suggestions.

⚠️ [IMPORTANT] The PR still removes the root .gitignore coverage for non-review workflow handoff files even though the issue/spec/implementation workflows continue to create and read root-level files such as issue_context.json, issue_comments.txt, implementation_summary.md, pr-metadata.json, branch-start-shas.json, validation-output.txt, and validation-error.txt. Local review outputs moved to a temp directory, but those other workflows were not migrated in this diff, so interrupted or local workflow runs will surface scratch artifacts as untracked files again. I am not adding a duplicate inline comment because the same .gitignore finding was already discussed in a resolved thread; the current diff still appears to leave that resolution incomplete.