refactor: split workflow-only skills from local skills

[Terry-Mao]

Closes #266

Summary

• 将 GitHub workflow-only skills 从 .agents/skills/ 迁移到 .github/skills/，保留 .agents/skills/ 作为本地/共享 skill 入口。
• 更新 workflow prompt、Python helper、validator、测试和文档，统一 workflow 专用 skill 路径。
• 更新 install.sh，同步 .github/skills，跳过 repo-local *-repo companion，并在升级时清理旧 .agents/skills workflow skill、迁移旧 companion。
• 同步 product raw/wiki 文档和 compile log，记录新的 agent 目录边界。

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/tests -p 'test_install_script.py'
• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/aicodingflow-tests
• PYTHONPYCACHEPREFIX=/tmp/aicodingflow-pycache python3 -m py_compile .github/scripts/*.py .github/skills/implement-specs/scripts/*.py .github/skills/review-pr/scripts/validate_review_json.py .github/skills/update-pr-review/scripts/*.py .github/skills/update-dedupe/scripts/*.py .github/skills/update-triage/scripts/*.py .agents/skills/resolve-merge-conflicts/scripts/*.py
• git diff --check
• $review-pr-local

[github-actions]

发现 1 个需要合并前修复的问题：安装脚本在同步本地 skills 后才清理 legacy workflow skills，会把本仓库自带的 repo-local companion 误迁移进目标仓库的 .github/skills。

[github-actions]

Applied requested changes on refactor/split-local-workflow-skills-266.

Summary:

Summary

• 先清理或迁移目标仓库 legacy .agents/skills workflow/companion，再同步当前 .agents/skills。
• 同步 .agents/skills 时排除 *-repo companion，避免源树残留 repo-local guidance 被安装到目标仓库。
• 增加安装脚本回归测试，覆盖源 .agents/skills 仍残留 legacy companion 的 fresh install 场景。

Validation

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

#268