✨ feat(skills): add skill_link symlink support + platform-agnostic prompt

- Add skill_link config option to create a symlink/junction from another
  platform's skills dir to agents_home/skills/ (e.g. ~/.claude -> ~/.agents)
- On Windows, falls back to directory junction when symlink requires admin
- Add _create_skills_symlink() with _is_junction() helper for Windows
- Update playbook.toml.example with skill_link documentation
- Fix templates/README.md prompt example to be platform-agnostic
- Add 3 tests: symlink creation, idempotency, absence when unconfigured

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

playbook.toml.example

@@ -40,7 +40,8 @@
 [install_skills]
 # mode = "list"                 # list|all
 # skills = ["brainstorming"]    # mode=list 时必填
-# agents_home = "~/.agents"     # Codex CLI；Claude Code 用 "~/.claude"；默认 ~/.agents
+# agents_home = "~/.agents"     # 部署目标；Codex CLI 用 "~/.agents"，Claude Code 用 "~/.claude"；默认 ~/.agents
+# skill_link = "~/.claude"      # 可选：在此目录下创建 skills/ 软链接指向 agents_home/skills/
 # no_backup = false             # 可选：跳过备份，直接删除旧 skill 后重装
 
 [format_md]