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

AGENTS.md

@@ -148,8 +148,8 @@ catalog.yaml               ←──   skiff add / fetch
     ▼         ▼
 ~/.cursor/skills/     project/.agents/skills/
 ~/.claude/skills/     project/.claude/skills/
-~/.codex/skills/      project/.opencode/skills/
-~/.config/opencode/skills/  project/.skills.yaml
+~/.codex/skills/      project/.skills.yaml
+~/.agents/skills/     （agents 标准目录，覆盖 OMP）
 ```
 
 ### Skill 三层分类
@@ -177,7 +177,7 @@ catalog.yaml               ←──   skiff add / fetch
 | Cursor      | `~/.cursor/skills/` | `.agents/skills/` |
 | Claude Code | `~/.claude/skills/` | `.claude/skills/` |
 | Codex       | `~/.codex/skills/`  | `.agents/skills/` |
-| OpenCode    | `~/.config/opencode/skills/` | `.opencode/skills/` |
+| Agents 标准 | `~/.agents/skills/` | `.agents/skills/`（与 cursor/codex 共用；覆盖 OMP） |
 
 
 ---