Create a named tmux session at wt_path and optionally send an initial command.
Returns (tmux_name, display_name). Tmux name is prefixed with {proj_name}-{wt_slug}-;
display_name is the user-visible part (what the user typed).
Create a new git worktree under repo_path for branch.
Runs hooks (env copy, post_create) and returns the new worktree path.
Returns a warning string if a hook failed (non-fatal).
Like refresh_workspace but with pre-computed worktree entries — avoids subprocess calls
on the caller’s thread. Used by the periodic background refresh path.