widget.forkSourceId took precedence over the fresh/resume logic on
EVERY (re)bind, so /clear in a fork pane re-forked the original
conversation instead of clearing, and /resume re-forked the same way.
The source is now copied into one-shot pane state and cleared after
the first successful fork spawn; later respawns operate on the pane's
own session.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
jpmschweitzerandClaude Fable 5
bbc6899df2