Roll back stored settings when a creation fails, and keep form ids unique

create_agent puts back the caps and inference settings it replaced if a
later store or the job insert fails, so a refused creation leaves nothing
behind for the name. The shared field components take an id prefix so the
create dialog and an open editor never share a DOM id. The queue check now
follows the hive roster check, the 422 is documented, and the copy says
empty rather than closed and that destroy keeps stored settings.

frontend/packages/swarm-ui/src/pages/CreateAgentForm.tsx

@@ -167,8 +167,8 @@ export function CreateAgentForm({
           <p class="create-agent-intro">
             Create a new agent's swarm-level identity. This only queues the job
             — check <Link href="/jobs">jobs</Link> to watch it settle. Resource
-            caps and inference are optional; leave them closed to keep the
-            defaults.
+            caps and inference are optional; leave them empty to set nothing. A
+            name set up before keeps the settings stored for it.
           </p>
           {hivesError && (
             <ApiErrorPanel
@@ -206,6 +206,7 @@ export function CreateAgentForm({
               <summary>resource caps</summary>
               <div class="create-agent-section-body">
                 <LimitsFields
+                  idPrefix="create-limits"
                   cpu={cpu}
                   memory={memory}
                   onCpu={setCpu}
@@ -216,7 +217,11 @@ export function CreateAgentForm({
             <details class="create-agent-section">
               <summary>inference</summary>
               <div class="create-agent-section-body">
-                <InferenceFields draft={inference} onChange={setInference} />
+                <InferenceFields
+                  idPrefix="create-inference"
+                  draft={inference}
+                  onChange={setInference}
+                />
               </div>
             </details>
             <Button