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_install_script.py

@@ -32,6 +32,7 @@ class InstallScriptTests(unittest.TestCase):
                 ".cursor/skills/skiff",
                 ".claude/skills/skiff",
                 ".codex/skills/skiff",
+                ".agents/skills/skiff",
             ):
                 link = home / relative
                 self.assertTrue(link.is_symlink(), relative)