feat(skiff): replace opencode target with agents standard dir

- targets: cursor, claude, codex, agents (~/.agents/skills, .agents/skills)
- agents dir covers OMP native discovery; drop omp/opencode targets
- project-level agents/cursor/codex share .agents/skills via idempotent symlinks
- migrate local state: remove ~/.config/opencode/skills links, reinstall into ~/.agents/skills

skills/skiff/SKILL.md

@@ -19,7 +19,7 @@ SSOT 固定在 `~/.skills/skills/<name>/`。内容通过 **symlink** 分发到
 
 ```bash
 skiff add --list
-skiff add <name> -a cursor -a claude -a codex -y
+skiff add <name> -a cursor -a claude -a codex -a agents -y
 ```
 
 `skiff add <name>` 默认安装到当前项目；只有用户明确需要所有项目使用时才加 `-g`。安装结果是指向 `~/.skills/skills/<name>/` 的软链，不要在 Agent 目录创建副本。