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

docs/swarm/inference.md

@@ -20,14 +20,15 @@ swarmctl agent inference clear iris
   agent's `availableModels` as the models its subagent `start` and
   `continue` calls may name, read on each call
   ([`subagent.md`](../tools/subagent.md#which-models-a-subagent-may-run-on)).
-  The web UI's model picker doesn't use it.
 - `get` shows each field, naming the unset ones (or says the swarm sets
   nothing for the agent), and then the model and effort the agent reports
   running on, each with where it comes from.
 - `clear` removes every field; the agent goes back to its own values.
 
 The full flag reference is [`swarmctl-cli.md`](../tools/swarmctl-cli.md). The
-swarm UI's agents page edits the same settings; see [UI](ui.md).
+swarm UI's agents page edits the same settings; see [UI](ui.md). The
+swarm is the only place to change an agent's model at runtime: its web UI
+shows the model but has no picker and no `/model` command.
 
 ## What an agent runs on
 
@@ -47,11 +48,14 @@ Each running value names where it comes from:
 | ------------ | -------------------------------------------------------------------------- |
 | `swarm`      | the value set here                                                         |
 | `agent_nix`  | `services.hyperhive.agent.model` or `effortLevel` in the agent's agent.nix |
-| `local_file` | a `/model` or `/effort` pick in the agent's web UI                         |
+| `local_file` | an `/effort` pick in the agent's web UI, or a `hyperhive-model` file       |
 | `default`    | the harness's built-in default (`haiku`, `medium`)                         |
 
 When the agent starts without a swarm value, its agent.nix model wins over
-its `/model` pick, and its `/effort` pick wins over its agent.nix effort.
+a `hyperhive-model` file, and its `/effort` pick wins over its agent.nix
+effort. A `hyperhive-model` file comes from a runtime model pick the
+agent's web UI no longer offers; the agent still reads it but never writes
+it.
 
 The agent reports these whenever they change, and again once a minute.
 swarm-controller keeps only what it has received since it started, so for up
@@ -63,13 +67,13 @@ Each agent reads its settings every 10 seconds and switches on the next
 read: its next turn runs on the new model or effort, and its web UI badge
 shows it. A turn already running keeps what it started with.
 
-- **Its own value is the one it runs without the swarm**, from a `/model`
-  or `/effort` pick in its web UI or from its agent.nix. Setting a swarm
+- **Its own value is the one it runs without the swarm**: its agent.nix, an
+  `/effort` pick in its web UI, or a `hyperhive-model` file. Setting a swarm
   value doesn't overwrite it.
 - **Clearing a field** (or the whole key) puts the agent back on its own
   value on the next read.
-- **A `/model` or `/effort` pick while the swarm sets that field** changes
-  the agent's own value, but the swarm value comes back on the next read.
+- **An `/effort` pick while the swarm sets effort** changes the agent's own
+  value, but the swarm value comes back on the next read.
 - **After a restart**, the agent runs its own values until its first read
   of its settings succeeds.