fix: prevent blank screen when clearing Capabilities field

The MarkdownEditor in the agent Configuration tab crashes when the
Capabilities field is fully cleared. The onChange handler converts empty
strings to null via (v || null), but the eff() overlay function then
returns that null to MDXEditor which expects a string, causing an
unhandled React crash and a blank screen.

Add a null-coalescing fallback (?? "") so MDXEditor always receives a
string.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/AgentConfigForm.tsx

@@ -515,7 +515,7 @@ export function AgentConfigForm(props: AgentConfigFormProps) {
             </Field>
             <Field label="Capabilities" hint={help.capabilities}>
               <MarkdownEditor
-                value={eff("identity", "capabilities", props.agent.capabilities ?? "")}
+                value={eff("identity", "capabilities", props.agent.capabilities ?? "") ?? ""}
                 onChange={(v) => mark("identity", "capabilities", v || null)}
                 placeholder="Describe what this agent can do..."
                 contentClassName="min-h-[44px] text-sm font-mono"