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

hive-agent/src/harness_state.rs

@@ -53,6 +53,27 @@ pub(crate) fn load_effort() -> Option<String> {
     }
 }
 
+/// The model this agent runs when the swarm sets none. Priority:
+/// `configured` (`HIVE_DEFAULT_MODEL`, from `services.hyperhive.agent.model`
+/// in agent.nix) > the persisted runtime override > [`DEFAULT_MODEL`].
+pub(crate) fn local_model(configured: Option<&str>) -> String {
+    configured.map_or_else(
+        || load_model().unwrap_or_else(|| DEFAULT_MODEL.to_owned()),
+        str::to_owned,
+    )
+}
+
+/// The effort this agent runs at when the swarm sets none. The inverse of
+/// [`local_model`]: a persisted operator pick wins over `configured`
+/// (`HIVE_DEFAULT_EFFORT`), so the runtime choice survives a harness restart
+/// (it only resets on `--purge`); nix is the default for a never-touched
+/// agent.
+pub(crate) fn local_effort(configured: Option<&str>) -> String {
+    load_effort()
+        .or_else(|| configured.map(str::to_owned))
+        .unwrap_or_else(|| DEFAULT_EFFORT.to_owned())
+}
+
 pub(crate) fn persist_effort(level: &str) -> std::io::Result<()> {
     let path = effort_file_path();
     if let Some(parent) = path.parent() {