fzf-pick the namespace when tsk switch is run with no name
`tsk switch` (and `tsk namespace switch`) now accept the name as
optional. When omitted, fzf-pick from existing namespaces with a
`<new>` sentinel for entering a fresh name at a prompt.
`tsk namespace create <name>` is unchanged (always requires a name).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>