Attachments are silently ignored when using the /fleet command

## 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:

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。

由索引模型根据 Issue 内容生成。