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

hive-agent/src/paths.rs

@@ -131,3 +131,8 @@ pub fn claude_dir() -> PathBuf {
     }
     PathBuf::from("/root/.claude")
 }
+
+/// Held by every test that points `HYPERHIVE_HARNESS_DIR` at a temp dir:
+/// cargo runs a crate's tests on parallel threads of one process.
+#[cfg(test)]
+pub(crate) static HARNESS_DIR_ENV: std::sync::Mutex<()> = std::sync::Mutex::new(());