Describe the feature or problem you'd like to solve

The --agent flag should be compatible with standard input (stdin) usage through a pipe.

Proposed solution

When run copilot with this way:

$ cat PROMPT.md | copilot --agent review

The error shown:

The --agent option can only be used in prompt mode (with --prompt).

I think the --agent flag should accept standard input (stdin) usage through a pipe without --prompt flag.

Example prompts or workflows

This should work.

$ cat PROMPT.md | copilot --agent review

Additional context

No response