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/Root.tsx

@@ -26,7 +26,7 @@ import { useTodos } from "./hooks/useTodos.js";
 import { fmtAge, fmtTokens } from "./lib/format.js";
 import { resolveDashboardBase } from "./lib/dashboardBase.js";
 import { submitPauseResume } from "./lib/pauseAction.js";
-import { postModel, postEffort } from "./lib/modelEffort.js";
+import { postEffort } from "./lib/modelEffort.js";
 import {
   postCancelTurn,
   postLogout,
@@ -220,8 +220,6 @@ export function Root() {
               statusLabel="… connecting"
               statusTone="neutral"
               model=""
-              availableModels={[]}
-              onSelectModel={() => {}}
               effort=""
               availableEfforts={[]}
               onSelectEffort={() => {}}
@@ -308,10 +306,7 @@ export function Root() {
             statusTooltip={primaryStatus.tooltip}
             model={state.model}
             resolvedModel={state.resolved_model ?? undefined}
-            availableModels={state.available_models}
-            onSelectModel={(name) => {
-              postModel(name).then(() => refresh());
-            }}
+            modelSource={state.model_source}
             effort={state.effort}
             availableEfforts={state.available_efforts}
             onSelectEffort={(level) => {