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

skiff/paths.py

@@ -20,17 +20,19 @@ AGENT_GLOBAL: dict[str, Path] = {
     "cursor": HOME / ".cursor" / "skills",
     "claude": HOME / ".claude" / "skills",
     "codex": HOME / ".codex" / "skills",
-    "opencode": HOME / ".config" / "opencode" / "skills",
+    # agents 标准目录：OMP 原生 canonical（agents provider），cursor/codex 项目级同路径
+    "agents": HOME / ".agents" / "skills",
 }
 
 AGENT_PROJECT: dict[str, str] = {
     "cursor": ".agents/skills",
     "claude": ".claude/skills",
     "codex": ".agents/skills",
-    "opencode": ".opencode/skills",
+    # 与 cursor/codex 共用 .agents/skills；软链幂等，同路径只写一次
+    "agents": ".agents/skills",
 }
 
-ALL_TARGETS = ("cursor", "claude", "codex", "opencode")
+ALL_TARGETS = ("cursor", "claude", "codex", "agents")
 
 
 def resolve_targets(target: str | None) -> list[str]: