Shared persistent state across all subconscious agents

Moved persistent_state from per-agent to a single shared BTreeMap on
Subconscious. All agents read/write the same state — surface's walked
keys are visible to observe and reflect, etc.

- Subconscious.state: shared BTreeMap<String, String>
- walked() derives from state["walked"] instead of separate Vec
- subconscious-state.json is now a flat key-value map
- All agent outputs merge into the shared state on completion
- Loaded on startup, saved after any agent completes

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

src/mind/mod.rs

@@ -258,7 +258,7 @@ impl Mind {
     }
 
     pub async fn subconscious_walked(&self) -> Vec<String> {
-        self.subconscious.lock().await.walked.clone()
+        self.subconscious.lock().await.walked()
     }
 
     pub async fn init(&self) {