fix(ui): correct context pruning default toggle state

Backend enables pruning by default (only skips when mode="off"), but
UI showed toggle OFF when no config was saved (null). Also fixed global
config dropdown defaulting to "off" instead of "auto".

ui/web/src/pages/agents/agent-detail/agent-advanced-dialog.tsx

@@ -52,7 +52,7 @@ export function AgentAdvancedDialog({ open, onOpenChange, agent, onUpdate }: Age
       } as ChatGPTOAuthRoutingConfig,
       wsSharing: (otherObj.workspace_sharing ?? {}) as WorkspaceSharingConfig,
       comp: a.compaction_config ?? {},
-      pruneEnabled: a.context_pruning != null,
+      pruneEnabled: a.context_pruning?.mode !== "off",
       prune: a.context_pruning ?? {},
       sbEnabled: a.sandbox_config != null,
       sb: a.sandbox_config ?? {},

ui/web/src/pages/config/sections/ai-defaults-section.tsx

@@ -207,7 +207,7 @@ export function AiDefaultsSection({ data, onSave, saving }: Props) {
           <div className="grid grid-cols-1 gap-4 sm:grid-cols-2">
             <div className="grid gap-1.5">
               <Label>{t("agents.pruning.mode")}</Label>
-              <Select value={pruning.mode ?? "off"} onValueChange={(v) => updateNested("contextPruning", { mode: v })}>
+              <Select value={pruning.mode ?? "auto"} onValueChange={(v) => updateNested("contextPruning", { mode: v })}>
                 <SelectTrigger><SelectValue /></SelectTrigger>
                 <SelectContent>
                   <SelectItem value="off">Off</SelectItem>