feat: add mycursor backend; drop myagents xiaohe entrypoint

Cursor Agent CLI wrapper (mycursor) with resume/session listing.
Product xiaohe CLI stays in xiaohe-api only — no dual entry.

tests/test_version.py

@@ -7,12 +7,6 @@ from myagents.commands import version as ver_mod
 
 
 class TestDescribeTree:
-    def test_legacy_runtime_tree(self) -> None:
-        tree = Path("/home/u/.xiaohe/runtime/v1.2.3/contrib/myagents")
-        mode, detail = ver_mod.describe_tree(tree)
-        assert mode == "legacy-runtime"
-        assert detail == "v1.2.3"
-
     def test_development_checkout(self, tmp_path: Path) -> None:
         repo = tmp_path / "repo"
         repo.mkdir()