Add named launch sets for repeated multi-session work

[jongio]

Problem

Dispatch supports selecting and launching multiple sessions, but the selection is temporary. Developers who run the same group of Copilot sessions for a repo or feature have to find and select those sessions every time.

Proposed solution

Add named launch sets backed by the existing config file. A user can select several sessions, save them as a named set, view saved sets from a small overlay, and launch every session in a set with the existing multi-launch path. If a saved session no longer exists, show it as missing and skip it at launch time.

User impact

Developers who split work across several Copilot sessions can restart their working set with one action. This brings Dispatch closer to tmux-style workspace pickers while staying within the current session model.

Acceptance criteria

• Users can save the current multi-selection as a named launch set.
• Users can list, rename, delete, and launch saved launch sets from the TUI.
• Launch sets are persisted in config and survive restart.
• Missing session IDs are shown clearly and skipped when launching a set.
• Tests cover config persistence and launch-set behavior.

Complexity & priority

Complexity: L | Priority: 4