Stage 3: scout guided flow — single-screen crew + standalone master step

• Slots: single-screen list seeded with ALL launchable agents (default model). Rows are selectable to Edit/Remove; "+ Add slot" adds one; "Done · N" finishes. Replaces the linear add-another loop.
• Master: separate step that now picks a STANDALONE master (its own agent + model via MasterAgent/MasterModel), chosen from installed text-generation-capable agents — not constrained to the worker slots. Only prompted when ≥2 slots.
• Task stays uniform across the whole crew (guided default); per-slot skills remain available via entire scout --edit.

Removes promptForSimpleReviewMaster + promptAddAnotherSlot; adds promptForStandaloneMaster, promptCrewSlot, promptSlotAction.