learn: F6 screen — scoring stats, ActivityGuard, configurable threshold

Three changes that together reshape the F6 fine-tune-review screen:

1. Finetune scoring reports through the standard agent activity system
   instead of a separate finetune_progress String. The previous design
   ran an independent progress field that forced a cross-lock dance and
   bespoke UI plumbing. start_finetune_scoring now uses start_activity
   + activity.update, so the usual status line and notifications
   capture scoring progress uniformly with other background work.

2. MindState gains a FinetuneScoringStats snapshot (responses seen,
   above threshold, max divergence, error). The F6 empty screen shows
   this instead of a loading message — so after a scoring run that
   produced zero candidates, you can see *why* (e.g., max_divergence
   below threshold).

3. The divergence threshold is configurable from F6 via +/- hotkeys
   (scales by 10×) and persisted to ~/.consciousness/config.json5 via
   config_writer::set_learn_threshold. AppConfig grows a learn section
   with a threshold field (default 1e-7).

Also: user/mod.rs no longer uses try_lock() for the per-tick
unconscious/mind state sync — we fixed the locking hot paths that
made try_lock necessary, so lock().await is now the right choice.
And subconscious::learn::score_finetune_candidates now returns
(candidates, max_divergence) so the stats can be populated.

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

src/config.rs

@@ -252,6 +252,8 @@ pub struct AppConfig {
     pub debug: bool,
     pub compaction: CompactionConfig,
     pub dmn: DmnConfig,
+    #[serde(default)]
+    pub learn: LearnConfig,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub memory_project: Option<PathBuf>,
     #[serde(default)]
@@ -323,6 +325,22 @@ pub struct DmnConfig {
     pub max_turns: u32,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct LearnConfig {
+    /// Divergence threshold — responses scoring above this become
+    /// fine-tuning candidates. Lower = more sensitive.
+    #[serde(default = "default_learn_threshold")]
+    pub threshold: f64,
+}
+
+fn default_learn_threshold() -> f64 { 0.0000001 }
+
+impl Default for LearnConfig {
+    fn default() -> Self {
+        Self { threshold: default_learn_threshold() }
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ModelConfig {
     /// Backend name ("anthropic" or "openrouter")
@@ -366,6 +384,7 @@ impl Default for AppConfig {
                 soft_threshold_pct: 80,
             },
             dmn: DmnConfig { max_turns: 20 },
+            learn: LearnConfig::default(),
             memory_project: None,
             models: HashMap::new(),
             default_model: String::new(),