feat: project directory tree + first-round snapshot

- cloud/tree.py: structured worktree tree (dirs first, ignore
  .git/node_modules, symlinks not followed, entry/depth
  truncation flags, text render for injection)
- GET /tasks/{id}/tree with /files auth + worktree resolution,
  max_depth 1..5, max_entries 1..500, escape 400, missing 404,
  cloud/cross-host 409
- MCP get_task_tree translating to the new endpoint
- worker first-round injection of a repo-checkout top-level
  snapshot (2 layers, 120 entries) after repo resolution,
  labelled as checkout snapshot not worktree; skipped for
  coordinator/resume/fork, silent when no host source
- docs/project-tree.md; tests for tree, endpoint, injection,
  MCP translation

cloud/mcp_server.py

@@ -21,6 +21,7 @@ contributor 等权限校验，403 原样透传。
 工具（不包管理端）：
   health、list_projects、list_tasks、get_task、create_task、
   cancel_task、get_task_cost、get_task_trace、get_task_files、
+  get_task_tree、
   get_task_result_full、retry_task、merge_task。
 create_task 只传 project_id + prompt + 可选 executor/model/
 effort/approval/priority，其余字段走服务端缺省；cancel_task
@@ -82,6 +83,11 @@ TOOLS = [
     _tool("get_task_files", "列/读任务 worktree 里的文件；不传 path 列顶层，传相对路径读文件或列目录。",
           {"task_id": {"type": "string"},
            "path": {"type": "string"}}, ["task_id"]),
+    _tool("get_task_tree", "取任务 worktree 的结构化目录树（只含名、类型、大小，不读文件内容）；不传 path 列 worktree 根，传相对路径列该子目录；max_depth 1..5 缺省 3，max_entries 1..500 缺省 200，截断时 truncated=True。",
+          {"task_id": {"type": "string"},
+           "path": {"type": "string"},
+           "max_depth": {"type": "integer"},
+           "max_entries": {"type": "integer"}}, ["task_id"]),
     _tool("get_task_result_full", "取单个任务的完整结果文本（result_text 只是摘要）。",
           {"task_id": {"type": "string"}}, ["task_id"]),
     _tool("retry_task", "重试一个失败/中断的任务（服务端状态机校验）。",
@@ -201,6 +207,20 @@ class MCPServer:
                 path += "?" + urllib.parse.urlencode(
                     {"path": args["path"]})
             return self._wrap(*self.transport("GET", path))
+        if name == "get_task_tree":
+            if not args.get("task_id"):
+                return True, {"error": "task_id required"}
+            query = {}
+            if args.get("path"):
+                query["path"] = args["path"]
+            if args.get("max_depth") is not None:
+                query["max_depth"] = args["max_depth"]
+            if args.get("max_entries") is not None:
+                query["max_entries"] = args["max_entries"]
+            path = "/tasks/" + args["task_id"] + "/tree"
+            if query:
+                path += "?" + urllib.parse.urlencode(query)
+            return self._wrap(*self.transport("GET", path))
         if name == "get_task_result_full":
             if not args.get("task_id"):
                 return True, {"error": "task_id required"}