fix: rollback optimistic update on failure and document remove option

- Capture previous defaultAgent before optimistic update and restore it
  in the catch block so the UI stays consistent on API failure
- Add "Remove the default agent" step to Slack configuration docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: amikofalvy

agents-docs/content/talk-to-your-agents/slack/configuration.mdx

@@ -29,6 +29,11 @@ The workspace default agent responds to all `@Inkeep` mentions and `/inkeep` com
 
     This agent handles all requests in channels without a channel-specific default.
   </Step>
+  <Step>
+    ### Remove the default agent (optional)
+
+    To clear the workspace default, open the dropdown and select **Remove default agent**. Channels without a channel-specific agent will no longer have a fallback.
+  </Step>
 </Steps>
 
 ## Channel defaults

agents-manage-ui/src/features/work-apps/slack/components/agent-configuration-card/index.tsx

@@ -144,6 +144,7 @@ export function AgentConfigurationCard() {
   const handleRemoveDefaultAgent = async () => {
     if (!teamId) return;
 
+    const previousAgent = defaultAgent;
     setDefaultAgent(null);
     setDefaultOpen(false);
     setSavingDefault(true);
@@ -154,6 +155,7 @@ export function AgentConfigurationCard() {
       toast.success('Default agent removed');
     } catch (error) {
       console.error('Failed to remove default agent:', error);
+      setDefaultAgent(previousAgent);
       toast.error('Failed to remove default agent');
     } finally {
       setSavingDefault(false);