swarm-ui: add required hive dropdown to CreateAgentPage

Closes #3434.

Agent creation had no way to record which hive an agent runs on.
POST /api/agents now requires a hive, so the roster fetched off
GET /api/hives backs a required SelectField here rather than a
free-text field. Single-hive swarms auto-select their only hive;
multi-hive swarms show a disabled placeholder and force an explicit
choice.

Rebuilt on top of the #3448 form kit (merged after this branch was
originally opened): reuses TextField/SelectField/Button instead of
page-scoped input chrome, and SelectField gains an optional disabled
placeholder option (needed for the loading/empty/multi-hive states
here, generalizes cleanly for future callers). Form goes back to a
column layout per mara's earlier visual feedback on this same page
(two fields of different natural width no longer line up in a row).

frontend/packages/swarm-ui/src/pages/CreateAgentPage.css

@@ -1,16 +1,19 @@
 /* <CreateAgentPage> — the create-agent form. Input/button chrome now
-   comes from the shared `ui/` form kit (`TextField`/`Button`); this
-   file only owns the page's own layout + copy. Reuses the same base16
-   slots (../theme.css) every other component draws from. */
+   comes from the shared `ui/` form kit (`TextField`/`SelectField`/
+   `Button`); this file only owns the page's own layout + copy. Column,
+   not row: with a second field (hive) added, a row layout put fields of
+   different natural widths on one baseline and looked misaligned —
+   mara: "make it a col". Reuses the same base16 slots (../theme.css)
+   every other component draws from. */
 .create-agent-intro {
   margin: 0 0 1.5em;
   color: var(--muted);
 }
 .create-agent-form {
   display: flex;
-  align-items: flex-end;
+  flex-direction: column;
+  align-items: flex-start;
   gap: 0.75em;
-  flex-wrap: wrap;
 }
 .create-agent-result {
   margin-top: 1em;

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

@@ -16,19 +16,35 @@
 // one path segment deep — a real bug, filed separately rather than
 // fixed here, but reason enough to avoid a nested route today.
 //
-// First real form in this package — its name field + submit button now
-// come from the shared `ui/` form kit (`TextField`/`Button`) rather than
-// page-scoped input/button chrome, so a hive-picker soon landing on this
-// same page has something to reuse instead of copying this page's CSS.
-import { useState } from 'preact/hooks';
+// `hive` field added because agent creation had no way to record which
+// hive an agent runs on. `POST /api/agents` now requires it, so the
+// roster fetched off `GET /api/hives` backs a required select here
+// rather than a free-text field — a spawn target has to be one of the
+// swarm's actual hives, same reasoning `Ident.parse` gets client-side
+// pattern validation for `name`.
+//
+// Its name field, hive select, and submit button all come from the
+// shared `ui/` form kit (`TextField`/`SelectField`/`Button`) rather than
+// page-scoped input/button chrome — see that kit's own comments for why.
+import { useEffect, useState } from 'preact/hooks';
 import { Link } from 'wouter-preact';
 import { ApiErrorPanel } from '@hive/shared/api-error-panel.js';
 import { readApiError, type ProblemDetails } from '@hive/shared/api-error.js';
 import { Panel } from '../ui/panel/Panel.js';
 import { TextField } from '../ui/text-field/TextField.js';
+import { SelectField, type SelectOption } from '../ui/select-field/SelectField.js';
 import { Button } from '../ui/button/Button.js';
 import './CreateAgentPage.css';
 
+// Mirrors swarm-controller's `HiveEntry` — same shape `OverviewPage`
+// consumes off `/api/hives/status`, but this page hits the plain
+// `/api/hives` roster (no status/freshness needed, just "what hives
+// exist to spawn into").
+interface HiveEntry {
+  name: string;
+  domain: string;
+}
+
 interface CreateAgentResponse {
   node_id: number;
 }
@@ -56,8 +72,37 @@ const NAME_PATTERN = '[a-z0-9\\-]{1,63}';
 
 export function CreateAgentPage() {
   const [name, setName] = useState('');
+  const [hive, setHive] = useState('');
+  // `null` = still loading, `[]` = loaded but empty (a real, if unusual,
+  // swarm state) — distinct from "not fetched yet" so the placeholder
+  // option's label can tell the two apart.
+  const [hives, setHives] = useState<HiveEntry[] | null>(null);
+  const [hivesError, setHivesError] = useState<ProblemDetails | null>(null);
   const [result, setResult] = useState<SubmitState>({ status: 'idle' });
 
+  // Loaded once, not re-fetched on submit: the roster changes rarely
+  // enough (a nix-level swarm config change) that staleness within one
+  // page visit isn't worth a second round-trip per keystroke/submit.
+  useEffect(() => {
+    (async () => {
+      const r = await fetch('/api/hives');
+      if (!r.ok) {
+        setHivesError(await readApiError(r));
+        return;
+      }
+      const data = (await r.json()) as HiveEntry[];
+      setHives(data);
+      // Single-hive swarms are the common case — default it rather than
+      // making the operator pick the only option. Multi-hive swarms get
+      // no default (the placeholder stays selected), so `required` on
+      // the select below forces an explicit choice per mara's scope.
+      if (data.length === 1) setHive(data[0].name);
+    })().catch((e: unknown) => setHivesError({ detail: String(e) }));
+  }, []);
+
+  const hiveOptions: SelectOption[] = (hives ?? []).map((h) => ({ value: h.name, label: h.name }));
+  const hivesReady = hives !== null && hives.length > 0;
+
   async function submit(e: Event) {
     e.preventDefault();
     setResult({ status: 'submitting' });
@@ -65,7 +110,7 @@ export function CreateAgentPage() {
       const r = await fetch('/api/agents', {
         method: 'POST',
         headers: { 'content-type': 'application/json' },
-        body: JSON.stringify({ name }),
+        body: JSON.stringify({ name, hive }),
       });
       if (!r.ok) {
         setResult({ status: 'error', problem: await readApiError(r) });
@@ -85,6 +130,7 @@ export function CreateAgentPage() {
         Create a new agent's swarm-level identity. This only queues the job — check{' '}
         <Link href="/jobs">jobs</Link> to watch it settle.
       </p>
+      {hivesError && <ApiErrorPanel context="failed to load the hive list" problem={hivesError} />}
       <form class="create-agent-form" onSubmit={submit}>
         <TextField
           id="agent-name"
@@ -95,7 +141,17 @@ export function CreateAgentPage() {
           required
           onInput={setName}
         />
-        <Button variant="primary" type="submit" disabled={result.status === 'submitting'}>
+        <SelectField
+          id="agent-hive"
+          label="hive"
+          value={hive}
+          onChange={setHive}
+          options={hiveOptions}
+          required
+          disabled={!hivesReady}
+          placeholder={!hives ? 'loading…' : hives.length === 0 ? 'no hives configured' : 'select a hive'}
+        />
+        <Button variant="primary" type="submit" disabled={result.status === 'submitting' || !hivesReady}>
           {result.status === 'submitting' ? 'creating…' : 'create'}
         </Button>
       </form>

frontend/packages/swarm-ui/src/ui/select-field/SelectField.tsx

@@ -18,6 +18,8 @@ export function SelectField({
   onChange,
   options,
   required,
+  disabled,
+  placeholder,
 }: {
   id: string;
   label: string;
@@ -25,6 +27,13 @@ export function SelectField({
   onChange: (value: string) => void;
   options: SelectOption[];
   required?: boolean;
+  disabled?: boolean;
+  // Rendered as a disabled, always-first `value=""` option — for a
+  // required select with no default (e.g. a multi-hive roster: no
+  // single right answer to pre-select), so the control shows something
+  // other than silently landing on whatever option happens to be first.
+  // Omit when the field always has a real default (`value` never `''`).
+  placeholder?: string;
 }) {
   return (
     <FormField label={label} htmlFor={id}>
@@ -33,8 +42,14 @@ export function SelectField({
         class="ui-form-control"
         value={value}
         required={required}
+        disabled={disabled}
         onChange={(e) => onChange((e.target as HTMLSelectElement).value)}
       >
+        {placeholder !== undefined && (
+          <option value="" disabled>
+            {placeholder}
+          </option>
+        )}
         {options.map((o) => (
           <option key={o.value} value={o.value}>
             {o.label}