feat: 图片改图 MCP 工具 image_edit

- imagegen.edit：POST multipart /v1/images/edits，输入图
  必须是工作区内 plain basename 且存在（上限 20MB），产出
  必须是新文件、绝不覆盖输入；响应解析与 generate 共用
  _save_image_response（url 或 b64_json、下载落盘、格式探测）。
- imagegen_mcp 新增 image_edit 工具；image-workflow 技能
  改图分支从“无专用工具”改为用 image_edit。
- 测试：multipart 体含提示词与原图字节、输入缺失/同名/
  路径逃逸拒绝、MCP 列表含 image_edit；全量相关 16 项过。
实测依据（2026-10-30）：中转站图片分组 key 调
gpt-image-2.5-flare edits 返回 200，URL 图为真实 PNG
1024x1024、909K，usage 含输入图 token（1659 image tokens）。

cloud/tests/test_imagegen.py

@@ -87,11 +87,55 @@ class ImageGenTests(unittest.TestCase):
         self.assertIn("404", str(ctx.exception))
         self.assertNotIn("k", str(ctx.exception))
 
+    def test_edit_multipart_writes_new_file(self):
+        with open(os.path.join(self._tmp.name, "in.png"), "wb") as fh:
+            fh.write(PNG_1x1)
+        seen = {}
+
+        def fake_multipart(url, key, body, content_type):
+            seen["url"] = url
+            seen["body"] = body
+            seen["content_type"] = content_type
+            self.assertTrue(url.endswith("/v1/images/edits"))
+            self.assertIn(b"Change the background", body)
+            self.assertIn(PNG_1x1, body)
+            return 200, {"data": [{"url": "https://cdn.example/e.png",
+                                   "revised_prompt": "blue bg"}],
+                         "usage": {"total_tokens": 2727}}
+
+        out = imagegen.edit(
+            "Change the background to blue", "in.png",
+            output_filename="out.png",
+            request_multipart=fake_multipart,
+            download=lambda url: PNG_1x1, cwd=self._tmp.name)
+        self.assertEqual(out["filename"], "out.png")
+        self.assertEqual(out["input_filename"], "in.png")
+        self.assertEqual(out["format"], "png")
+        self.assertTrue(seen["content_type"].startswith(
+            "multipart/form-data; boundary="))
+        # Input untouched, output is a separate file.
+        with open(os.path.join(self._tmp.name, "in.png"), "rb") as fh:
+            self.assertEqual(fh.read(), PNG_1x1)
+        self.assertTrue(os.path.exists(
+            os.path.join(self._tmp.name, "out.png")))
+
+    def test_edit_missing_input_and_same_output_rejected(self):
+        with self.assertRaises(ImageGenError):
+            imagegen.edit("x", "nope.png", cwd=self._tmp.name)
+        with open(os.path.join(self._tmp.name, "in.png"), "wb") as fh:
+            fh.write(PNG_1x1)
+        with self.assertRaises(ImageGenError):
+            imagegen.edit("x", "in.png", output_filename="in.png",
+                          cwd=self._tmp.name)
+        with self.assertRaises(ImageGenError):
+            imagegen.edit("x", "../in.png", cwd=self._tmp.name)
+
     def test_mcp_tools_list_and_call(self):
         listed = handle_request({"jsonrpc": "2.0", "id": 1,
                                  "method": "tools/list"})
         names = [t["name"] for t in listed["result"]["tools"]]
         self.assertIn("image_generate", names)
+        self.assertIn("image_edit", names)
         called = handle_request(
             {"jsonrpc": "2.0", "id": 2, "method": "tools/call",
              "params": {"name": "image_generate",