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/server.py

@@ -74,6 +74,9 @@ from cloud.peer_messages import ensure_followups_table as _peer_ensure_followups
 from cloud.peer_messages import peer_message_prefix as _peer_prefix
 from cloud.peer_messages import row_get as _row_get
 from cloud.marker_tools import parse_spawn_defaults as _parse_spawn_defaults
+from cloud.tree import DEFAULT_MAX_DEPTH as _TREE_DEFAULT_DEPTH
+from cloud.tree import DEFAULT_MAX_ENTRIES as _TREE_DEFAULT_ENTRIES
+from cloud.tree import build_tree as _build_tree
 from cloud.automations import cancel_automation as _auto_cancel
 from cloud.automations import create_automation as _auto_create
 from cloud.automations import get_automation as _auto_get
@@ -2316,6 +2319,8 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
                 self._task_timeline(user_id, rest[: -len("/timeline")])
             elif rest.endswith("/files") and method == "GET":
                 self._task_files(user_id, rest[: -len("/files")], query)
+            elif rest.endswith("/tree") and method == "GET":
+                self._task_tree(user_id, rest[: -len("/tree")], query)
             elif rest.endswith("/browser/state") and method == "GET":
                 self._browser_state(user_id,
                                    rest[: -len("/browser/state")])
@@ -5214,6 +5219,61 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
         self._send_json(200, {"task_id": task_id, "path": rel,
                               "size": size, "content": content})
 
+    def _task_tree(self, user_id, task_id, query):
+        """GET /tasks/{id}/tree：结构化目录树（不读文件内容）。
+
+        与 /files 同款权限与 worktree 解析：viewer+ 可读，
+        cloud 任务与跨机 worktree 一律 409，解析不到时 404。
+        ?path=<相对路径>：不传列 worktree 根；传目录列该目录。
+        ?max_depth：1..5，缺省 3；?max_entries：1..500，缺省
+        200。非法整数一律 400，不静默回落缺省。路径逃逸
+        400，不存在或非目录 404。条数到顶时 truncated=True，
+        被 max_depth 截断的目录 children=[] 且
+        children_truncated=True，空目录则是 children=[] 且
+        children_truncated=False，两者可区分。
+        """
+        task, err = self._thread_task(user_id, task_id, "read")
+        if err is not None:
+            self._send_json(err[0], {"error": err[1]})
+            return
+        worktree, werr = _task_worktree_for_files(self.server, task)
+        if werr is not None:
+            self._send_json(werr[0], {"error": werr[1]})
+            return
+        try:
+            max_depth = int((query.get("max_depth")
+                             or [str(_TREE_DEFAULT_DEPTH)])[0])
+            max_entries = int((query.get("max_entries")
+                               or [str(_TREE_DEFAULT_ENTRIES)])[0])
+        except (TypeError, ValueError):
+            self._send_json(400, {"error":
+                                  "max_depth/max_entries must be integers"})
+            return
+        if not 1 <= max_depth <= 5 or not 1 <= max_entries <= 500:
+            self._send_json(400, {"error":
+                                  "max_depth must be 1..5 and "
+                                  "max_entries must be 1..500"})
+            return
+        rel = (query.get("path") or [""])[0]
+        try:
+            tree = _build_tree(worktree, path=rel,
+                               max_depth=max_depth,
+                               max_entries=max_entries)
+        except ValueError:
+            self._send_json(400, {"error": "path escapes task worktree"})
+            return
+        except FileNotFoundError:
+            self._send_json(404, {"error": "no such path"})
+            return
+        except NotADirectoryError:
+            self._send_json(404, {"error": "path is not a directory"})
+            return
+        tree = dict(tree)
+        tree["task_id"] = task_id
+        tree["max_depth"] = max_depth
+        tree["max_entries"] = max_entries
+        self._send_json(200, tree)
+
     def _browser_worktree_for(self, user_id, task_id):
         """Auth + worktree resolution shared by browser endpoints.