feat: use vellum for herdr workspace picker

[dkarter]

Motivation

Use Vellum for the Herdr workspace popup while keeping the existing Television channel available.

Summary of changes

• Install dkarter/vellum through Mise.
• Add Vellum's official Herdr workspace palette.
• Send Vellum's TUI output back to the popup terminal and focus the selected workspace.

Testing

• Run task sync.
• Open the Herdr workspace picker and confirm the workspace list is visible.
• Select a workspace and confirm Herdr focuses it.
• Run task ci:run.

Dependencies/Special considerations

None.