Preserve valid agent retry progress

tests/test_git.py

@@ -53,4 +53,34 @@ def test_discard_changes_restores_clean_disposable_workspace(tmp_path) -> None:
     workspace.discard_changes()
 
     assert tracked.read_text() == "<?php\nreturn [];\n"
-    assert not untracked.exists()
+    assert not untracked.exists()
+
+
+def test_checkpoint_restores_last_valid_changes(tmp_path) -> None:
+    workspace = GitWorkspace(settings(workspace_root=tmp_path), 4)
+    workspace.path.mkdir()
+    subprocess.run(["git", "init"], cwd=workspace.path, check=True, capture_output=True)
+    subprocess.run(
+        ["git", "config", "user.email", "test@example.invalid"],
+        cwd=workspace.path,
+        check=True,
+    )
+    subprocess.run(
+        ["git", "config", "user.name", "Test"],
+        cwd=workspace.path,
+        check=True,
+    )
+    tracked = workspace.path / "config.php"
+    tracked.write_text("<?php\nreturn [];\n")
+    subprocess.run(["git", "add", "."], cwd=workspace.path, check=True)
+    subprocess.run(["git", "commit", "-m", "base"], cwd=workspace.path, check=True, capture_output=True)
+    tracked.write_text("<?php\nreturn ['valid'];\n")
+    added = workspace.path / "test.php"
+    added.write_text("<?php\n")
+    checkpoint = workspace.checkpoint()
+    tracked.write_text("<?php\nreturn [broken;\n")
+
+    workspace.restore_checkpoint(checkpoint)
+
+    assert tracked.read_text() == "<?php\nreturn ['valid'];\n"
+    assert added.exists()