Attachments are silently ignored when using the /fleet command

首先，在带有附件和 /fleet 命令的 Copilot agent 会话中复现该问题，然后将其与附件能够正常传递的普通消息进行比较。跟踪命令专用消息的处理位置，并确定附件是否可以转发，或者是否必须在运行开始前触发警告。完成标准是附件可供 Fleet 使用，或用户在运行开始前收到明确的警告。

Summary

When a message includes attachments (for example an image, a file or folder
reference, selected code, or large pasted content) and that message uses the
/fleet command, the attachments are not delivered to the agent. Fleet runs
using only the typed text, and the user is given no indication that the
attached context was dropped.

Steps to reproduce

1. Open an agent session (Copilot).
2. Add one or more attachments to the message (e.g. paste an image, attach a
   file, or add a file/folder reference).
3. Start the message with the /fleet command, e.g.
   /fleet analyze the attached screenshot and split the work.
4. Send the message.

Expected behavior

Either:

• the attachments are made available to Fleet just as they are for a normal
  message, or
• if attachments cannot be used with /fleet, the user is clearly told before
  the run starts (so they can resend without /fleet or adjust their request).

Actual behavior

• The attachments are silently discarded.
• Fleet proceeds using only the typed prompt text.
• There is no warning, error, or other signal that the attachments were ignored.

Impact

• The agent can produce confident but wrong results because it never received
  context the user explicitly attached and referred to.
• The loss is invisible: users reasonably assume attachments were included,
  since they are included for normal (non-/fleet) messages.
• This is most damaging when the request directly depends on the attachment
  (e.g. "analyze this screenshot", "refactor the attached file").

Possible resolutions

1. Support forwarding attachments to Fleet so /fleet behaves like a normal
   message with attachments.
2. If attachments can't be supported, surface a clear, upfront message when a
   /fleet message includes attachments, instead of silently dropping them.

Notes

• Reproducible with the /fleet command specifically; normal messages deliver
  attachments as expected.
• Severity is "silent data/context loss," which is easy to miss and hard to
  diagnose after the fact.

Environment

• VS Code version:
• OS: