fix: task files endpoint resolves tenant docker worktrees

GET /tasks/{id}/files previously returned 409 for all docker tasks
on the grounds that tenant docker shares /app/work per project.
Production inspection shows the worker creates a per-task
host-worktrees/<task-id> subdirectory under the tenant work dir
(62 such dirs under the wwww tenant), so per-task reads are
attributable after all: the endpoint now resolves tenant docker
tasks via tenants/<project>/work/host-worktrees/<task-id> (local
worktree as fallback), never serves the shared /app/work mount
itself, and still 409s cloud/cross-host tasks. Docs and the
harness2_tasks boundary notes are updated to match.

cloud/tests/test_server_task_files.py

@@ -37,6 +37,8 @@ class TaskFilesTestCase(unittest.TestCase):
         self.worktrees = os.path.join(self.tmp.name, "host-worktrees")
         os.makedirs(self.worktrees)
         self.server.host_worktrees_dir = self.worktrees
+        self.tenants = os.path.join(self.tmp.name, "tenants")
+        self.server.tenants_root = self.tenants
         self.port = self.server.server_address[1]
         self.thread = threading.Thread(
             target=self.server.serve_forever, daemon=True)
@@ -61,6 +63,14 @@ class TaskFilesTestCase(unittest.TestCase):
         Path(self.tmp.name, "outside.txt").write_text("secret\n",
                                                        encoding="utf-8")
         self._task("task-docker-1", executor="docker")
+        tenant_wt = os.path.join(
+            self.tenants, self.project["id"], "work",
+            "host-worktrees", "task-docker-1")
+        os.makedirs(tenant_wt)
+        Path(tenant_wt, "report.md").write_text("Total: 85.15\n",
+                                                encoding="utf-8")
+        self._task("task-docker-nodir", executor="docker")
+        self._task("task-cloud-1", executor="cloud")
         self._task("task-nowt-1", executor=None)
 
     def tearDown(self):
@@ -150,9 +160,23 @@ class TaskFilesTestCase(unittest.TestCase):
         status, body = self._files("task-files-1", "big.txt")
         self.assertEqual(status, 413, body)
 
-    def test_docker_task_409_and_auth(self):
+    def test_tenant_docker_task_files(self):
+        # 租户 Docker 任务走 tenants 下的任务级子目录，不再 409。
         status, body = self._files("task-docker-1")
+        self.assertEqual(status, 200, body)
+        self.assertEqual([e["path"] for e in body["entries"]],
+                         ["report.md"])
+        status, body = self._files("task-docker-1", "report.md")
+        self.assertEqual(status, 200, body)
+        self.assertEqual(body["content"], "Total: 85.15\n")
+
+    def test_docker_task_without_worktree_404_and_cloud_409(self):
+        status, _ = self._files("task-docker-nodir")
+        self.assertEqual(status, 404)
+        status, body = self._files("task-cloud-1")
         self.assertEqual(status, 409, body)
+
+    def test_auth(self):
         status, _ = self._req("GET", "/tasks/task-files-1/files")
         self.assertEqual(status, 401)
         status, _ = self._files("task-files-1", key=self.outsider_key)