fix(backend): create_commit 直接引用经典空树 SHA（W1-C2 #165，ADR-0054）

arbiter/backend.py

@@ -114,16 +114,16 @@ def delete_ref(self, ref: str) -> None:
             raise NotFoundError(f"ref 不存在: {ref}")
         raise InfraError(f"deleteRef HTTP {status}: {payload.get('message', '')}")
 
+    # 经典空树 SHA（git mktree </dev/null 的恒定值）——租约/台账 commit 不携带
+    # 任何文件内容，元数据全在 commit message。e2e 实测（.github#206 演习，
+    # conductor run 32493680834）：POST /git/trees 对空 body {}/{"tree":[]}
+    # 一律 422 "Invalid tree info"（GitHub 不接受创建空树），该路径在真实 API
+    # 上从未成功过——单测走 LocalGitBackend 故未暴露。直接引用空树 SHA 即可。
+    EMPTY_TREE_SHA = "4b825dc642cb6eb9a060e54bf8d69288fbee4904"
+
     def create_commit(self, message: str, parent: str | None = None) -> str:
         """空树 commit，message 内嵌 JSON（租约/台账载体，ADR-0054 §2）。"""
-        status, payload = self._request(
-            "POST", f"/repos/{self._repo}/git/trees", {})
-        if status != 201:
-            raise InfraError(f"createTree HTTP {status}: {payload.get('message', '')}")
-        tree = payload.get("sha")
-        if not tree:
-            raise InfraError(f"createTree 未返回 sha: {payload}")
-        body = {"message": message, "tree": tree}
+        body = {"message": message, "tree": self.EMPTY_TREE_SHA}
         if parent:
             body["parents"] = [parent]
         status, payload = self._request(

tests/test_github_backend_wire.py

@@ -0,0 +1,66 @@
+"""GitHubRefBackend 请求形状回归测试（W1-C2 补充）。
+
+背景：create_commit 原实现先 POST /git/trees 空 body 建"空树"，但 GitHub 对
+空树创建一律 422 "Invalid tree info"（e2e .github#206 演习实测，
+conductor run 32493680834）——该路径在真实 API 上从未成功，单测因走
+LocalGitBackend 而未暴露。本测试在 HTTP 请求层（mock _request）锁定请求形状，
+防再次引入"创建空树"调用。
+"""
+
+import unittest
+from unittest.mock import patch
+
+from arbiter.backend import GitHubRefBackend
+
+
+def make_backend():
+    return GitHubRefBackend(token="t-test", repo="o/r")
+
+
+class TestCreateCommitWire(unittest.TestCase):
+    def test_create_commit_uses_canonical_empty_tree_no_trees_call(self):
+        """create_commit 不得调用 /git/trees；commit 请求体 tree=经典空树 SHA。"""
+        calls = []
+
+        def fake_request(method, path, body=None):
+            calls.append((method, path, body))
+            if path.endswith("/git/commits"):
+                return 201, {"sha": "c0ffee"}
+            raise AssertionError(f"意外调用: {method} {path}")
+
+        b = make_backend()
+        with patch.object(b, "_request", side_effect=fake_request):
+            sha = b.create_commit("lease-meta-json", parent=None)
+        self.assertEqual(sha, "c0ffee")
+        self.assertEqual(len(calls), 1, "create_commit 应只发一次 POST /git/commits")
+        method, path, body = calls[0]
+        self.assertEqual(method, "POST")
+        self.assertTrue(path.endswith("/git/commits"))
+        self.assertEqual(body["tree"], GitHubRefBackend.EMPTY_TREE_SHA)
+        self.assertEqual(body["message"], "lease-meta-json")
+        self.assertNotIn("parents", body)  # 无父提交（新租约）
+
+    def test_create_commit_with_parent(self):
+        calls = []
+
+        def fake_request(method, path, body=None):
+            calls.append((method, path, body))
+            return 201, {"sha": "beef"}
+
+        b = make_backend()
+        with patch.object(b, "_request", side_effect=fake_request):
+            sha = b.create_commit("renew", parent="aaa")
+        self.assertEqual(sha, "beef")
+        self.assertEqual(calls[0][2]["parents"], ["aaa"])  # 接管=以旧租约为父
+
+    def test_empty_tree_sha_is_the_git_canonical_constant(self):
+        # 4b825dc…是空树的恒定 SHA（git mktree </dev/null），任何 git 环境可复验；
+        # 若此断言失败说明有人改动了常量——需附带真实 API 证据再改
+        self.assertEqual(
+            GitHubRefBackend.EMPTY_TREE_SHA,
+            "4b825dc642cb6eb9a060e54bf8d69288fbee4904",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()