swarm: per-agent inference settings (model, effort, subagent_models) in KV

swarm-controller keeps each agent's inference settings in a new
`agent-inference` KV bucket, one key per agent, and is its only writer
(`GET`/`PUT`/`DELETE /api/agents/{name}/inference`; `swarmctl agent
inference get|set|clear`). Every field is optional. Each agent reads its
own key every 10 s through a direct-get grant on that key alone, and a
set `model` or `effort` replaces the Bus value, so the next turn runs on
it and the badge follows.

The swarm value is applied without persisting it: `Bus::set_model` and
`set_effort` are split into a persisting setter and a non-persisting
`apply_*`, and the poller uses the latter. Otherwise the swarm value
would overwrite `hyperhive-model`/`hyperhive-effort`, and clearing it, or
a restart, would no longer find the agent's own value. An unset field
changes nothing; a field cleared after being applied goes back to the
local value (`harness_state::local_model`/`local_effort`, the same
resolution `Bus::new` uses), read again at the clear.

`subagent_models` is validated, stored and shown by `get`; no agent
applies it yet.

Refs #5034

docs/swarm/inference.md

@@ -0,0 +1,68 @@
+# Per-agent inference settings
+
+The swarm can set the model and reasoning effort each agent's turns run on,
+plus the models it may start subagents on. A field you set wins over the
+agent's own value; a field you leave unset keeps the agent on its own value,
+so an agent you set nothing for runs as it does today.
+
+## Setting them
+
+```sh
+swarmctl agent inference set iris --model opus --effort high
+swarmctl agent inference get iris
+swarmctl agent inference clear iris
+```
+
+- `set` changes only the flags you pass; the other fields keep what's
+  stored. A hive must declare the agent.
+- `--effort` is one of `low`, `medium`, `high`, `xhigh`, `max`.
+- `--subagent-models` takes a comma-separated list. It's stored and shown,
+  but agents don't apply it yet.
+- `get` shows each field, naming the unset ones.
+- `clear` removes every field; the agent goes back to its own values.
+
+The full flag reference is [`swarmctl-cli.md`](../tools/swarmctl-cli.md).
+
+## When an agent picks a change up
+
+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
+  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.
+- **After a restart**, the agent runs its own values until its first read
+  of its settings succeeds.
+
+<details><summary>Implementation</summary>
+
+The settings live in the `agent-inference` KV bucket, one key per agent,
+which swarm-controller alone writes:
+
+```json
+{ "model": "opus", "effort": "high", "subagent_models": ["haiku", "sonnet"] }
+```
+
+Every field is optional, and `{}` reads the same as no key.
+
+| method   | path                            | body                                                                                                                                 |
+| -------- | ------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------ |
+| `GET`    | `/api/agents/<agent>/inference` | —; answers the stored value, `{}` when none                                                                                          |
+| `PUT`    | `/api/agents/<agent>/inference` | the whole value, replacing the stored one; the controller refuses unknown fields, an unknown effort, or a model name with whitespace |
+| `DELETE` | `/api/agents/<agent>/inference` | —; answers 204                                                                                                                       |
+
+An agent's own queue credential grants it
+`$JS.API.DIRECT.GET.KV_agent-inference.$KV.agent-inference.<agent>`, the
+direct read of its own key, and nothing else in this bucket — the same shape
+as its schedule key ([`schedules.md`](schedules.md)).
+
+The harness (`hive-agent`'s `swarm_inference`) applies a set field to the
+in-memory model or effort without writing `hyperhive-model` or
+`hyperhive-effort`, which keep the agent's own value.
+
+</details>