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_select.py

@@ -78,7 +78,7 @@ class SelectorTests(unittest.TestCase):
                 "ack",
                 "builtin",
                 installed=False,
-                readonly_status="全局: cursor,claude,codex,opencode",
+                readonly_status="全局: cursor,claude,codex,agents",
             )
         ]
         screen = Mock()
@@ -101,7 +101,7 @@ class SelectorTests(unittest.TestCase):
             if call.args[0] == 3
         )
         self.assertIn("[ ] ack", title)
-        self.assertIn("全局: cursor,claude,codex,opencode", title)
+        self.assertIn("全局: cursor,claude,codex,agents", title)
         self.assertNotIn("初始化", title)
 
     def test_selector_renders_description_on_indented_second_line(self) -> None:
@@ -370,7 +370,7 @@ class SelectCommandTests(unittest.TestCase):
             other.mkdir()
             global_dirs = {
                 target: root / target / "skills"
-                for target in ("cursor", "claude", "codex", "opencode")
+                for target in ("cursor", "claude", "codex", "agents")
             }
             for directory in global_dirs.values():
                 directory.mkdir(parents=True)
@@ -385,7 +385,7 @@ class SelectCommandTests(unittest.TestCase):
                 note = cli._global_installation_note(
                     "ack",
                     expected,
-                    ["cursor", "claude", "codex", "opencode"],
+                    ["cursor", "claude", "codex", "agents"],
                 )
 
         self.assertEqual(note, "全局: cursor；全局同名冲突: claude")