* feat(tui): add isolated btw conversations
Add /btw with inline-question support, isolated non-resumable session forks, background main-run routing, and safe return behavior.
Refs #637
* test(tui): cover btw return controls
Preserve drafts when Ctrl+C is pressed during an active BTW run and cover returning to the parent through the documented /btw toggle.
* Address BTW isolation review feedback
* Reject explicit resumes of side sessions
* Harden BTW isolation lifecycle
