Agent Skills: worktree-setup

Create the codesmith implementation worktree and branch via worktrunk. Resolves the branch name from the parent ticket's primary tag (per tag-recipes/{tag}.md Branch prefix section) and the parallel-safety determination (single worktree for sequential plans; one worktree per plan for parallel-safe multi-plan). Invokes `wt switch -c <branch>`. Returns the worktree path and branch name to the caller.