The RequestConflictResolution Func was declared on 5 VMs and hand-threaded shell->details->merge-section->diff->merge-modal. Replaced with a DI-singleton IMergeCoordinator (MergeCoordinator holder; shell wires its Handler at composition, breaking the shell<->island cycle). Invokers (MergeModal, DetailsIsland, WorktreesOverview) depend on the interface; the two pass-through VMs (DiffModal, MergeSection) drop the seam entirely. No behavior change; conflict-seam + batch tests rewired to assert via the coordinator.
Collapses the ~10 per-modal Show*Modal Func callbacks (wired separately on the shell and the lists island) into one IDialogService + WindowDialogService impl. Removes the RepoImport/WorktreesOverview dialog construction duplicated across MainWindow and ListsIslandView, plus the Confirm/Error dialogs duplicated in both code-behinds. Shell/lists Open* commands now route through an injected Dialogs handle (propagated shell->lists); the per-list worktrees overview also wires conflict resolution now, matching the global one. No VM ctor changes (Dialogs is a settable seam), so no test-fake impact.
