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/README.md

@@ -23,6 +23,8 @@ grafana, the controller, authelia, …) →
   graph, agent creation. → [below](#swarm-controller)
 - **scheduled prompts** — named prompts for one or more agents at set times,
   kept by the swarm and fired in each agent's container. → [`schedules.md`](schedules.md)
+- **inference settings** — the model and effort the swarm sets for an agent,
+  over its own. → [`inference.md`](inference.md)
 - **swarm UI** — the operator's day-to-day surface, on the swarm apex,
   `admins` only. → [`ui.md`](ui.md)
 - **shared services** — one forge, homeserver, SSO, queue and metrics/logs