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/agent/tools/memory.rs

@@ -20,7 +20,7 @@ fn get_f64(args: &serde_json::Value, name: &str) -> Result<f64> {
     args.get(name).and_then(|v| v.as_f64()).context(format!("{} is required", name))
 }
 
-async fn cached_store() -> Result<std::sync::Arc<tokio::sync::Mutex<Store>>> {
+async fn cached_store() -> Result<std::sync::Arc<crate::Mutex<Store>>> {
     Store::cached().await.map_err(|e| anyhow::anyhow!("{}", e))
 }