feat: 图片生成 MCP 工具 image_generate（中转站图片分组）

- cloud/imagegen.py：POST /v1/images/generations，解析 url 或
  b64_json，下载落盘到工作区；basename 校验防路径逃逸；
  缺 IMAGE_GEN_API_KEY 时明确报未配置，绝不回退任务自己
  的分组 2 key（分组 2 allow_image=False，回退只会 404）。
- cloud/imagegen_mcp.py：stdio MCP，工具 image_generate。
- worker --imagegen-mcp：租户 codex-home 播种脚本与
  [mcp_servers.imagegen]，容器启动前写 env（系统级
  IMAGE_GEN_* + 代理 + NO_PROXY 网关 host）。
- image-workflow 技能：有 image_generate 时用它生成，
  无工具或报未配置时明说无法生成；改图暂无专用工具。
- 测试：imagegen 9 项 + worker 播种/env 3 项。
实测依据（2026-10-30）：分组 4 key 调 gpt-image-2.5-flare
返回 200，URL 图为真实 PNG 1024x1024；此前分组 4 账号
schedulable=False 时全模型 404，调度开启后恢复。
部署注意（主机侧，不在代码里）：env 需配 IMAGE_GEN_API_KEY；
Tinyproxy 白名单需加 qinglanapi.top（下载返回图 URL 用）；
autoscaler 需加 --imagegen-mcp。

cloud/imagegen_mcp.py

@@ -0,0 +1,167 @@
+"""Stdio MCP server exposing image_generate to codex.
+
+Protocol: JSON-RPC 2.0 over stdio, one JSON object per line.
+Logs go to stderr only -- anything on stdout would corrupt the
+JSON-RPC stream.
+
+This file is deliberately thin: all provider logic lives in
+``imagegen.py``. It can run two ways:
+- inside the repo: ``python3 -m cloud.imagegen_mcp`` (imports
+  ``cloud.imagegen``);
+- copied flat into a tenant codex-home next to ``imagegen.py``
+  (imports plain ``imagegen`` from its own directory).
+The ``ensure_codex_home_imagegen`` helper in imagegen.py does the
+copying; keys are read from the MCP server process environment
+by imagegen.py, never from the tool arguments.
+"""
+
+import json
+import os
+import sys
+
+try:
+    from cloud.imagegen import ImageGenError, generate
+except ImportError:  # flat copy in a tenant codex-home
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    from imagegen import ImageGenError, generate  # noqa: E402
+
+SERVER_NAME = "imagegen"
+SERVER_VERSION = "0.1.0"
+
+_TOOLS = [
+    {
+        "name": "image_generate",
+        "description": (
+            "Generate one image from a text prompt via the relay's "
+            "image group and save it as a file in the current working "
+            "directory (the task worktree). Returns the absolute path, "
+            "format, dimensions when detectable, byte size and the "
+            "relay usage block. The file is a real generated image, "
+            "never a placeholder: on any failure the tool errors "
+            "instead of writing a stand-in file. Verify the returned "
+            "path with shell/PIL before reporting success."
+        ),
+        "inputSchema": {
+            "type": "object",
+            "properties": {
+                "prompt": {"type": "string",
+                           "description": "Image description in detail"},
+                "output_filename": {"type": "string",
+                                    "description": "Plain basename "
+                                    "to save as, e.g. 'hero.png'. "
+                                    "Default: generated-<epoch>.png"},
+                "size": {"type": "string", "default": "1024x1024",
+                         "description": "1024x1024, 1536x1024, "
+                         "1024x1536, or auto"},
+                "model": {"type": "string",
+                          "description": "Image model override; "
+                          "default is the IMAGE_GEN_MODEL env "
+                          "or gpt-image-2.5-flare"},
+            },
+            "required": ["prompt"],
+        },
+    },
+]
+
+
+def _err(code, message):
+    return {"code": code, "message": message}
+
+
+def handle_request(request, generate_fn=None):
+    """Handle one JSON-RPC request dict. Returns a response dict or
+    None for notifications (no id). Pure function for testability:
+    the stdio loop passes the real generate, tests pass fakes."""
+    generate_fn = generate_fn or generate
+    if not isinstance(request, dict):
+        return {"jsonrpc": "2.0", "id": None,
+                "error": _err(-32700, "request is not an object")}
+    method = request.get("method")
+    req_id = request.get("id")
+    is_notification = "id" not in request
+
+    if method == "initialize":
+        if is_notification:
+            return None
+        return {"jsonrpc": "2.0", "id": req_id,
+                "result": {
+                    "protocolVersion": request.get("params", {}).get(
+                        "protocolVersion", "2024-11-05"),
+                    "capabilities": {"tools": {}},
+                    "serverInfo": {"name": SERVER_NAME,
+                                   "version": SERVER_VERSION}}}
+    if method in ("notifications/initialized", "notifications/cancelled"):
+        return None
+    if method == "ping":
+        if is_notification:
+            return None
+        return {"jsonrpc": "2.0", "id": req_id, "result": {}}
+    if method == "tools/list":
+        if is_notification:
+            return None
+        return {"jsonrpc": "2.0", "id": req_id,
+                "result": {"tools": _TOOLS}}
+    if method == "tools/call":
+        if is_notification:
+            return None
+        params = request.get("params") or {}
+        name = params.get("name")
+        args = params.get("arguments") or {}
+        try:
+            if name == "image_generate":
+                result = generate_fn(
+                    args.get("prompt"),
+                    output_filename=args.get("output_filename"),
+                    size=args.get("size", "1024x1024"),
+                    model=args.get("model"))
+            else:
+                return {"jsonrpc": "2.0", "id": req_id,
+                        "error": _err(-32602, "unknown tool: %r" % name)}
+        except Exception as exc:  # noqa: BLE001 - tool errors are data
+            # ImageGenError messages are key-free by construction
+            # in imagegen.py; other exceptions are rendered as
+            # type + message, never with env or headers.
+            return {"jsonrpc": "2.0", "id": req_id,
+                    "result": {"content": [{"type": "text",
+                                            "text": "error: %s" % exc}],
+                               "isError": True}}
+        return {"jsonrpc": "2.0", "id": req_id,
+                "result": {"content": [{"type": "text",
+                                        "text": json.dumps(
+                                            result, ensure_ascii=False)}],
+                           "isError": False}}
+    if is_notification:
+        return None
+    return {"jsonrpc": "2.0", "id": req_id,
+            "error": _err(-32601, "method not found: %r" % method)}
+
+
+def main():
+    for line in sys.stdin:
+        line = line.strip()
+        if not line:
+            continue
+        try:
+            request = json.loads(line)
+        except json.JSONDecodeError as exc:
+            sys.stdout.write(json.dumps(
+                {"jsonrpc": "2.0", "id": None,
+                 "error": _err(-32700, "parse error: %s" % exc)}) + "\n")
+            sys.stdout.flush()
+            continue
+        try:
+            response = handle_request(request)
+        except Exception as exc:  # noqa: BLE001 - server must not die
+            print("imagegen_mcp: handler error: %s" % exc, file=sys.stderr)
+            if "id" in request:
+                response = {"jsonrpc": "2.0", "id": request["id"],
+                            "error": _err(-32603, "internal error")}
+            else:
+                response = None
+        if response is not None:
+            sys.stdout.write(json.dumps(response) + "\n")
+            sys.stdout.flush()
+
+
+if __name__ == "__main__":
+    main()