refactor(workflows): move handoff files to temp dir

[Terry-Mao]

Summary

• Move create-spec, create-implementation, and PR comment response handoff files into runner temp directories
• Pass context, metadata, summaries, snapshots, validation logs, and implementation-output paths explicitly through workflow prompts and validators
• Update workflow contract tests and product documentation to reflect temp handoff behavior

Validation

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

Notes

This removes the remaining need for root .gitignore coverage for these workflow scratch files. The repository worktree now carries only real implementation/spec diffs while handoff artifacts are uploaded from the runner temp directory.

[github-actions]

发现 1 个需要合并前修复的问题。PR 把 spec、implementation 和 PR comment response 的 handoff 行为改成 runner temp 目录，但长期产品知识的权威 raw 文档仍保留旧说法；这会让编译 wiki 与 source-of-truth 冲突。

[github-actions]

发现 1 个需要合并前修复的问题：产品 wiki 写入了新的 handoff 路径事实，但对应 raw 权威文档没有同步，后续 wiki 查询或重新编译会继续得到冲突信息。未发现需要单独指出的安全问题。

[github-actions]

仍需处理已有未解决 review thread（docs/product/wiki/concepts/agent-workflow-boundaries.md:42）。当前 diff 虽然同步了 implementation 和 PR comment response 的 raw 文档，但该行仍把 Spec workflow 的 context、metadata、validation logs 与 summary handoff 一起写成已确认的 .codex-runtime/handoff/ 行为；docs/product/raw/spec-workflow.md 仍没有确认 spec agent 输出文件契约，而且当前 create-spec workflow 实际只上传 issue_context、issue_comments、pr-metadata，没有 validation logs 或 summary handoff。为避免重复同一条未解决 inline thread，这里不再创建新的 inline comment；请先同步/限定 spec raw 与 wiki 表述后再合并。

[github-actions]

未发现需要阻塞合并的 correctness、workflow contract 或 security 问题。已特别核对 handoff 路径在 create-spec、create-implementation、respond-to-pr-comment workflow 与相关验证/提交脚本之间的传递一致性；先前关于 raw/wiki 不一致的评论在当前 diff 中也已同步修正。