fix(change-window): accept empty Git reference transactions

loopx/capabilities/repository_change_window/git_hook.py

@@ -992,6 +992,11 @@ def _reference_transaction_introduces_commit(
         ReferenceTransactionPhase.PREPARED,
     }:
         return False
+    # Git can invoke admission hooks for a zero-update transaction (including
+    # Git 2.55's pull path). No refs means no new commit to guard; provider
+    # integrity and previous-hook delegation remain owned by the caller.
+    if not hook_stdin:
+        return False
     if not hook_stdin.strip():
         raise RepositoryChangeWindowError(
             f"reference-transaction {phase.value} phase requires at least one ref update"

tests/capabilities/test_change_window_git_fast_path.py

@@ -35,9 +35,12 @@
     )
 
     def git(repo, *args, check=True):
-        return subprocess.run(
-            ["git", "-C", str(repo), *args], check=check, capture_output=True, text=True
+        result = subprocess.run(
+            ["git", "-C", str(repo), *args], check=False, capture_output=True, text=True
         )
+        if check:
+            assert result.returncode == 0, result.stdout + result.stderr
+        return result
 
     git(remote, "config", "user.name", "Synthetic User")
     git(remote, "config", "user.email", "user@example.invalid")
@@ -130,7 +133,7 @@
             hook_args=[phase],
             hook_stdin=row,
         )
         assert result["ok"] and not result["policy_evaluated"]
         assert result["decision"] is None
     previous = tmp_path / "previous" / "reference-transaction"
     previous.write_text("#!/bin/sh\necho previous-failure >&2\nexit 7\n")
@@ -141,7 +144,7 @@
         hook_args=["prepared"],
         hook_stdin=row,
     )
     assert result["exit_code"] == 7 and result["status"] == "previous_hook_failed"
     managed = repo / ".git/loopx/repository-change-window/hooks/pre-push"
     managed.write_text("#!/bin/sh\nexit 0\n")
     result = owner.run_git_hook_provider(
@@ -163,6 +166,41 @@
     assert result.returncode == 0, result.stderr
 
 
+@pytest.mark.parametrize("phase", ["preparing", "prepared"])
+def test_empty_transaction_is_noop_but_keeps_hook_guards(
+    installed, tmp_path, monkeypatch, phase
+):
+    repo, _remote, _git, _policy = installed
+
+    def unexpected(*args, **kwargs):
+        raise AssertionError("empty transaction evaluated the time policy")
+
+    monkeypatch.setattr(owner, "evaluate_policy", unexpected)
+    args = {
+        "repo_path": repo,
+        "runtime_root": tmp_path / "runtime",
+        "event": "reference-transaction",
+        "hook_args": [phase],
+        "hook_stdin": b"",
+    }
+    result = owner.run_git_hook_provider(**args)
+    assert result["ok"] and result["previous_hook_invoked"]
+    assert not result["guarded_change"] and not result["policy_evaluated"]
+    assert result["decision"] is None
+    assert (tmp_path / "phases").read_text().splitlines() == [phase]
+    assert (tmp_path / "payloads").read_bytes() == b""
+    # An empty batch is legal; a nonempty malformed row is not.
+    with pytest.raises(ValueError, match="at least one ref update"):
+        owner.run_git_hook_provider(**{**args, "hook_stdin": b" \n"})
+    previous = tmp_path / "previous" / "reference-transaction"
+    previous.write_text("#!/bin/sh\nexit 7\n")
+    result = owner.run_git_hook_provider(**args)
+    assert result["status"] == "previous_hook_failed" and result["exit_code"] == 7
+    managed = repo / ".git/loopx/repository-change-window/hooks/reference-transaction"
+    managed.write_text(managed.read_text() + "# modified\n")
+    assert owner.run_git_hook_provider(**args)["status"] == "provider_drift"
+
+
 def test_old_valid_hook_generation_requires_explicit_refresh(installed):
     repo, _remote, _git, policy = installed
     state_path = repo / ".git/loopx/repository-change-window/provider.json"