second_spawn_on_same_branch_is_rejected asserted the same pre-#77
outright-rejection contract as the unit test fixed in 0e20baf, just at
the e2e/session level — also broken by 49ab7e9's branch-collision
suffixing and also already failing on main.
Renamed to second_spawn_on_same_branch_gets_suffixed and rewritten to
assert the actual behavior: the second spawn succeeds on a `-2`
suffixed branch and directory, and the first worktree is untouched.