locks: add process-wide lock hold time tracking

TrackedMutex and TrackedRwLock wrappers that record hold durations
by source location using #[track_caller]. Stats written to
~/.consciousness/lock-stats.json every second, sorted by max hold time.

Re-exported as crate::Mutex so all locks are instrumented. To disable,
swap the re-export back to tokio::sync::Mutex.

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

src/mind/subconscious.rs

@@ -410,7 +410,7 @@ impl Subconscious {
     /// Late-init: push the output tool onto each agent's tool list.
     /// Called after Subconscious is wrapped in Arc<Mutex<>> so the
     /// closure can capture a reference back.
-    pub fn init_output_tool(&mut self, self_arc: std::sync::Arc<tokio::sync::Mutex<Self>>) {
+    pub fn init_output_tool(&mut self, self_arc: std::sync::Arc<crate::Mutex<Self>>) {
         for agent in &mut self.agents {
             let Some(ref mut auto) = agent.auto else { continue };
             let sub = self_arc.clone();