Mirrors the web spawn form: worktree/branch controls sit behind a new
"Advanced options" disclosure line (bracket [+]/[-], toggled by Enter
or Space, closed by default) instead of always showing the checkbox
and branch rows. The header states whether a worktree will be created
so #91's off-by-default for a worktree row is visible without
expanding. Collapsed fields drop out of Tab order; expanding only adds
rows below the disclosure, never moving the ones above it.