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/README.md

@@ -84,6 +84,8 @@ declarations.
   sealed?** → [`swarm/bao.md`](swarm/bao.md).
 - **How does an agent get a prompt at set times, and why doesn't it fire
   twice after downtime?** → [`swarm/schedules.md`](swarm/schedules.md).
+- **How do I set an agent's model or effort from the swarm, and what happens
+  to its own pick?** → [`swarm/inference.md`](swarm/inference.md).
 
 ## Scheduler, CI, observability