spawn_task chip: starting via 'cloud' drops the prompt/brief from the spawned session

Describe the bug

When a session uses the spawn_task tool to create a background-task chip (a suggested follow-up with a title, tldr, and a full prompt body), the chip can apparently be started two ways: via a worktree (local) session, or via a "cloud" session.

Starting a chip via the worktree avenue correctly delivers the full prompt body as the new session's context.

Starting the same chip via the cloud avenue does not carry the prompt body into the spawned session — even when the prompt is long and fully self-contained (file paths, line numbers, verified root cause, explicit constraints). The cloud session starts with effectively no brief, as if only the chip's title/tldr (or nothing) came through.

To Reproduce

1. In a Claude Code session, have Claude call spawn_task with a detailed, self-contained prompt (not just a short tldr).
2. In the chip that appears in the UI, choose to start it in a cloud session.
3. Observe that the spawned cloud session has no knowledge of the detailed plan/brief from the prompt field.
4. Compare against starting the same chip via the worktree option, where the brief is present.

Expected behavior

Starting a chip via either avenue (worktree or cloud) should deliver the same prompt content to the spawned session.

Additional context

Workaround found: re-spawning the same chip and starting it via worktree instead of cloud delivers the brief correctly. Filing this so the cloud-start code path can be checked for whatever step forwards prompt into the new session.