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.css

@@ -13,8 +13,8 @@
 /* mara, live, on a screenshot of this row: "badges and interactive
    elements on same row have inconsistent size." Real cause: Badge.css's
    `.ui-badge-interactive` grows to a 2.75em WCAG touch-target floor
-   (model/effort pickers, pause) while a plain display `<span>` badge
-   (alive, state, ctx, cost — no `onClick`) stays compact, so the row
+   (effort picker, pause) while a plain display `<span>` badge
+   (model, alive, state, ctx, cost — no `onClick`) stays compact, so the row
    reads as two different pill sizes even though every badge here is
    the same `<Badge>` component. Scoped to this row rather than
    Badge.css itself — display-only badges elsewhere (e.g. the dashboard)