Echo ds4-agent prompts and slash commands in scrollback

[rowantrollope]

Summary

Echo accepted ds4-agent input back into terminal scrollback with an explicit ds4-agent> marker.

Why

Terminal scrollback should preserve the cause-and-effect trail of an interactive session. Once input is accepted, it stops being a live editable prompt and becomes part of the user's visible activity record. Keeping submitted prompts and slash commands in scrollback makes it clear which output belongs to which input, especially after command output, long model responses, interruptions, or copied terminal logs.

This keeps two UI concepts distinct:

• the live ds4-agent> prompt is where the next input is edited
• the echoed ds4-agent> ... line is the durable record of accepted input

The command echo remains terminal-only; slash commands are still kept out of the model transcript/session history.

Changes

• Replace the previous red * user-prompt echo with ds4-agent> <prompt>.
• Add the same terminal-only echo for recognized slash commands before their output.
• Keep unknown slash commands out of scrollback so they can still be restored for editing.

Testing

• make ds4-agent