Naming a worktree-isolated subagent same as an existing worktree dir collides

When spawning a background agent with git-worktree isolation, giving it a name that matches an existing worktree directory can cause a branch/worktree collision: the parent session shell got switched off its own branch onto the newly created agent branch, and a second agent spawned in the same batch produced no worktree/branch/commit at all. No work was lost (everything was committed), but the state looked tangled. Give isolation:worktree agents names distinct from any existing worktree directory, and verify each spawned agent actually created its own worktree via git worktree list before assuming parallel isolation held.