Attachments are silently ignored when using the /fleet command

Start by reproducing the issue in a Copilot agent session with an attachment and the /fleet command, then compare it with a normal message where attachments are delivered. Trace where the command-specific message is handled and determine whether attachments can be forwarded or must trigger a pre-run warning. Done means attachments are available to Fleet or users receive a clear warning before the run starts.

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: