mind: MindTriggered trait for background scoring flows

Mind's impl had accumulated ~50 lines of setup glue per scoring flow
(memory, memory-full, finetune): snapshot config, clone handles,
resolve context, spawn task, route results back through BgEvent,
write stats. The shape was identical; only the middle changed.

Introduce the MindTriggered trait:

    pub trait MindTriggered {
        fn trigger(&self);
    }

Each flow becomes a struct next to its scoring code that owns its
dependencies and a JoinHandle (behind a sync Mutex for interior
mutability):

    subconscious::learn::MemoryScoring    (Score, ScoreFull)
    subconscious::learn::FinetuneScoring  (ScoreFinetune)

Mind holds one of each and dispatches in one line:

    MindCommand::Score         => self.memory_scoring.trigger(),
    MindCommand::ScoreFull     => self.memory_scoring.trigger_full(),
    MindCommand::ScoreFinetune => self.finetune_scoring.trigger(),

Each struct picks its own trigger semantics — memory scoring is
no-op-if-running (!handle.is_finished()); finetune is abort-restart.

Falls out:

 - BgEvent / bg_tx / bg_rx disappear entirely. Tasks write directly
   to their slice of MindState and call agent.state.changed.notify_one()
   to wake the UI. The bg_rx arm in Mind's select loop is gone.

 - agent.state.memory_scoring_in_flight was duplicating
   shared.scoring_in_flight via BgEvent routing; now the JoinHandle
   alone tells us, and shared.scoring_in_flight is written directly
   by the task for the UI.

 - start_memory_scoring / start_full_scoring / start_finetune_scoring
   methods on Mind are deleted; Mind no longer knows the setup shape
   of any scoring flow.

 - FinetuneScoringStats moves from mind/ to subconscious/learn.rs
   next to the function that produces it.

No behavior change — same flows, same trigger points, same semantics.

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

src/user/mod.rs

@@ -504,6 +504,7 @@ async fn run(
                     keep
                 });
             }
+
             app.mind_state = Some(ms.clone());
         }
         app.walked_count = mind.subconscious_walked().await.len();