chore(neuron): default tracing filter to info (was info,neuron=debug)

Production deployments that want neuron-internal debug detail (e.g.
trim_device_pool's per-clear-kv line, slab inserts/drops) override
RUST_LOG explicitly via systemd. Defaulting to debug for the whole
neuron target produced a lot of journal volume that wasn't useful in
the common case.

beast already sets RUST_LOG=debug in
/etc/systemd/system/neuron.service.d/local.conf, so beast's verbosity
is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/src/main.rs

@@ -70,8 +70,7 @@ async fn main() -> Result<()> {
     tracing_subscriber::fmt()
         .with_writer(std::io::stderr)
         .with_env_filter(
-            EnvFilter::try_from_default_env()
-                .unwrap_or_else(|_| EnvFilter::new("info,neuron=debug")),
+            EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info")),
         )
         .init();