agents: remove the hive-level /model control; the swarm editor sets the model

The per-agent web UI no longer sets the model: the `/model` slash
command, the model picker, `POST /api/model`, `Bus::set_model` and the
`hyperhive-model` writer are gone. The model chip is display-only and
shows where the model comes from (`model_source` in `/api/state`). The
model is set on the swarm (swarm UI agents page, `swarmctl agent
inference set`).

The read side is unchanged: with no swarm value an agent runs its
agent.nix model, else an existing `hyperhive-model` file, else haiku.
Existing files are not touched.

With the picker gone, its model list goes too: `available_models` in
`/api/state`, the ACP model-list filter, and the harness's global
`HIVE_AVAILABLE_MODELS`. The subagent daemon keeps its own
`HIVE_AVAILABLE_MODELS` from mcp.nix, and the `availableModels`
assertion stays.

`/effort` (command, picker, route, file) is unchanged.

Refs #5034

frontend/packages/agent/src/components/StatusChips.tsx

@@ -1,6 +1,7 @@
 // <StatusChips> — the agent page's status row: the consolidated status
 // badge (alive/thinking/paused/etc., with pause/resume + cancel-turn in
-// its dropdown), the model + effort pickers, and context/cost usage.
+// its dropdown), the model (display-only: the swarm sets it), the effort
+// picker, and context/cost usage.
 // This is the concrete fix for the design guide's own named anti-example
 // (the old page's model/effort *pickers* lived in the `⋯` overflow menu
 // while the current model/effort only showed as a disconnected chip) —
@@ -13,6 +14,7 @@
 import { useRef, useState } from "preact/hooks";
 import { Badge, type BadgeTone } from "@hive/shared/badge.js";
 import { Dropdown, type DropdownOption } from "@hive/shared/dropdown.js";
+import type { ModelSource } from "../types.js";
 import "./StatusChips.css";
 
 export interface StatusChipsProps {
@@ -25,8 +27,8 @@ export interface StatusChipsProps {
    * operator can tell what an alias resolved to. `undefined` before any
    * turn has completed. */
   resolvedModel?: string;
-  availableModels: string[];
-  onSelectModel: (name: string) => void;
+  /** Where `model` comes from (`/api/state` `model_source`). */
+  modelSource?: ModelSource;
   effort: string;
   availableEfforts: string[];
   onSelectEffort: (level: string) => void;
@@ -50,10 +52,11 @@ export interface StatusChipsProps {
   onLogout: () => Promise<void>;
 }
 
-const MODEL_DESCRIPTIONS: Record<string, string> = {
-  haiku: "fast",
-  sonnet: "balanced",
-  opus: "powerful",
+const SOURCE_LABEL: Record<ModelSource, string> = {
+  swarm: "set on the swarm",
+  agent_nix: "from agent.nix",
+  local_file: "from its own earlier pick",
+  default: "the built-in default",
 };
 const EFFORT_DESCRIPTIONS: Record<string, string> = {
   low: "",
@@ -235,8 +238,7 @@ export function StatusChips({
   statusTooltip,
   model,
   resolvedModel,
-  availableModels,
-  onSelectModel,
+  modelSource,
   effort,
   availableEfforts,
   onSelectEffort,
@@ -263,13 +265,17 @@ export function StatusChips({
         onNewSession={onNewSession}
         onLogout={onLogout}
       />
-      <Picker
+      <Badge
         label="model"
         value={model}
-        options={availableModels}
-        descriptions={MODEL_DESCRIPTIONS}
-        onSelect={onSelectModel}
-        title={resolvedModel ? `resolved to ${resolvedModel}` : undefined}
+        title={
+          [
+            modelSource ? SOURCE_LABEL[modelSource] : null,
+            resolvedModel ? `resolved to ${resolvedModel}` : null,
+          ]
+            .filter(Boolean)
+            .join(" · ") || undefined
+        }
       />
       {availableEfforts.length ? (
         <Picker