Steadier workspace, and “create” makes real pages

We fixed two issues that could make the workspace feel stuck. A chat response that couldn't be written now fails gracefully and ends the round, instead of retrying in a tight loop that slowed everything down. And deleting a node now returns immediately when it succeeds, instead of appearing to hang for up to a minute.

We also sharpened what happens when you ask an assistant to create something: it now makes a proper mesh node — a Markdown page by default, or the right specialized node (a Space, agent, and so on) — rather than dropping a stray .txt file.