Slash commands in the prompter, starting with Export session

The prompter already had two mention namespaces: @ for skills and files, and the model and mode chips beside it. A slash namespace gives Realm somewhere to put actions that belong to the session rather than to the agent running in it.

Export session is the first. It writes the transcript out in a form you can read outside Realm. That is the request that had been arriving most often, and the one hardest to satisfy by hand.

Realm's commands are listed separately from the harness's own. A palette that mixes them makes it impossible to tell which will still be there if you change model.