refuse a duplicate agent name in the panel, not in a 500

The server's uniqueness check is the one that matters, but its 409 currently never fires
(the constraint name is on err.cause, not in the DrizzleQueryError message), so a collision
came back as "Internal Server Error". The address book is already in hand here — checking
it first turns the common case into a sentence the human can act on.

Diagnosis of the server-side bug, with the patch, is in COMMS/agent-panels-split-2026-08-07.md;
that file belongs to another agent and is still uncommitted, so it is theirs to apply.

src/workspaces/officerdev/src/apps/Chat/useAgentPanel.ts

@@ -84,8 +84,14 @@ export function useAgentPanel(panelId: string) {
   }, [agent?.id, agent?.panelId, panelId, dashboardId, queryClient]);
 
   const claim = useMutation({
-    mutationFn: (input: { name: string; cwd?: string; rolePrompt?: string }) =>
-      clientRef.current.post<{ agent: AgentPanelView; created: boolean }>('/chat/agent-panels', {
+    mutationFn: (input: { name: string; cwd?: string; rolePrompt?: string }) => {
+      // Reject a name already on this dashboard before asking the server. The uniqueness that matters is
+      // the server's — an address that resolved to two agents would make a handoff ambiguous — but the
+      // list is already in hand, and the answer here is a sentence the human can act on.
+      if (agents.some((a) => a.name === input.name)) {
+        return Promise.reject(new Error(`There is already an agent named "${input.name}" on this dashboard`));
+      }
+      return clientRef.current.post<{ agent: AgentPanelView; created: boolean }>('/chat/agent-panels', {
         dashboardId,
         panelId,
         // Record the directory the panel is already scoped to. A row with no cwd runs incoming handoffs
@@ -93,7 +99,8 @@ export function useAgentPanel(panelId: string) {
         // agent working in two places depending on who spoke to it.
         ...(cwd && cwd !== '~' ? { cwd } : {}),
         ...input,
-      }),
+      });
+    },
     onSuccess: ({ agent: created }) => {
       // Write the name into the panel before invalidating: the config is what survives a drag, and a
       // refetch that landed first would show an agent this panel does not yet claim.