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

nix/host-modules/swarm-nats.nix

@@ -739,6 +739,9 @@ in
                 # `CONSUMER.CREATE` (a watch's create subject carries no key,
                 # so it would cover every agent's) and no `$KV` write.
                 "--agent-token-publish-subject ${lib.escapeShellArg "\$\$JS.API.DIRECT.GET.KV_agent-schedules.\$\$KV.agent-schedules.{agent}"}"
+                # Its own key of the `agent-inference` KV bucket
+                # (`swarm_queue_client::agent_inference`), on the same terms.
+                "--agent-token-publish-subject ${lib.escapeShellArg "\$\$JS.API.DIRECT.GET.KV_agent-inference.\$\$KV.agent-inference.{agent}"}"
                 "--store-cert-role ${lib.escapeShellArg authCertRole}"
               ];
               # The store identity arrives by `LoadCredential` and is named