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/lib/modelEffort.ts

@@ -1,4 +1,4 @@
-// POST `/api/model` / `/api/effort` — same-origin (this agent's own
+// POST `/api/effort` — same-origin (this agent's own
 // backend), so a plain `fetch` is safe unlike pause/resume (see
 // pauseAction.ts). `redirect: 'manual'` + the broad "ok" check below is
 // ported from app.js unchanged — the endpoints don't actually redirect
@@ -34,6 +34,5 @@ async function post(
   }
 }
 
-export const postModel = (name: string) => post("api/model", "model", name);
 export const postEffort = (level: string) =>
   post("api/effort", "effort", level);

frontend/packages/agent/src/lib/pauseAction.ts

@@ -6,7 +6,7 @@
 // hive-c0re doesn't need to grow CORS headers for it — the tradeoff is
 // an opaque, unreadable response, which is fine here since the caller
 // doesn't need to read it, only trigger a state refresh afterward (same
-// shape as `postModel`/`postEffort` in modelEffort.ts). `credentials:
+// shape as `postEffort` in modelEffort.ts). `credentials:
 // 'include'` matches the cookie-sending behavior of the plain <form>
 // submit this replaces.
 //