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

tests/test_create_workflow.py

@@ -83,13 +83,13 @@ class CreateWorkflowTests(unittest.TestCase):
             ".cursor/skills/skiff",
             ".claude/skills/skiff",
             ".codex/skills/skiff",
+            ".agents/skills/skiff",
         ):
             link = self.home / relative
             self.assertTrue(link.is_symlink(), relative)
             self.assertEqual(link.resolve(), expected)
-        self.assertIn("cursor", first.stdout)
-        self.assertIn("claude", first.stdout)
-        self.assertIn("codex", first.stdout)
+        for agent in ("cursor", "claude", "codex", "agents"):
+            self.assertIn(agent, first.stdout)
 
     def test_update_pulls_skills_home(self) -> None:
         with (