refactor(Mountain): Consolidate WindServiceHandler to WindServiceHandlers atoms

Merge the deprecated singular `WindServiceHandler` module into the plural `WindServiceHandlers` atoms:
- Delete 24 files from `WindServiceHandler/` (21 were dead duplicates)
- Re-implement 3 live handlers as atoms: `extensions:install`, `extensions:uninstall`, `nativeHost.showOpenDialog`
- Move extension handlers to `WindServiceHandlers/Extension/` (Install, Uninstall, NotifyCocoonDeltaExtensions, UserExtensionDirectory, VsixPathFromArgs)
- Move dialog handlers to `WindServiceHandlers/NativeDialog/` (ShowOpenDialog, ParseDialogFilters, DialogFilter)
- Add 9 new notification atoms: ExtensionActivated, ExtensionDeactivated, ExtensionHostMessage, LanguagesSetDocumentLanguage, ProgressStart, ProgressReport, ProgressEnd, WebviewReady, WorkspaceApplyEdit
- Update MountainVinegRPCService to dispatch notifications via atoms (note: outputChannel.* already dispatched from Batch 9)

The consolidation eliminates code duplication and centralizes all IPC dispatch in the plural module. New notification atoms enable Wind/Sky to receive extension lifecycle events and progress updates from Cocoon.

Author: NikolaRHristov