unconscious: release lock during slow spawn work

Split trigger() into phases so the Unconscious mutex is only held briefly:
- reap_finished(): check handles, restore completed autos
- select_to_spawn(): pick agents, take their autos out
- prepare_spawn(): slow work (Store::load, query, Agent::new) - NO LOCK
- complete_spawn()/abort_spawn(): store results back

Previously held the lock for 28+ seconds during Store::load and query
execution. Now lock hold time should be milliseconds.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/mind/mod.rs

@@ -346,7 +346,19 @@ impl Mind {
                         s.unc_idle = true;
                     }
                     loop {
-                        unc.lock().await.trigger().await;
+                        // Phase 1: quick work under lock
+                        let to_spawn = {
+                            let mut guard = unc.lock().await;
+                            guard.reap_finished();
+                            guard.select_to_spawn()
+                        };
+                        // Phase 2: slow work outside lock
+                        for (idx, name, auto) in to_spawn {
+                            match crate::mind::unconscious::prepare_spawn(&name, auto).await {
+                                Ok(result) => unc.lock().await.complete_spawn(idx, result),
+                                Err(auto) => unc.lock().await.abort_spawn(idx, auto),
+                            }
+                        }
                         // Check if conscious became active
                         if *unc_rx.borrow() { break; }
                         // Brief yield to not starve other tasks