fix: replace implicit app inference with explicit selection for Skills import and sync

Skills import previously inferred app enablement from filesystem presence,
causing incorrect multi-app activation when the same skill directory existed
under multiple app paths. Now the frontend submits explicit app selections
via ImportSkillSelection, and schema migration preserves a snapshot of
legacy app mappings to avoid lossy reconstruction.

Also adds reconciliation to sync_to_app (removes disabled/orphaned symlinks)
and MCP sync_all_enabled (removes disabled servers from live config).

src/App.tsx

@@ -712,17 +712,14 @@ function App() {
             <UnifiedSkillsPanel
               ref={unifiedSkillsPanelRef}
               onOpenDiscovery={() => setCurrentView("skillsDiscovery")}
+              currentApp={activeApp === "openclaw" ? "claude" : activeApp}
             />
           );
         case "skillsDiscovery":
           return (
             <SkillsPage
               ref={skillsPageRef}
-              initialApp={
-                activeApp === "opencode" || activeApp === "openclaw"
-                  ? "claude"
-                  : activeApp
-              }
+              initialApp={activeApp === "openclaw" ? "claude" : activeApp}
             />
           );
         case "mcp":