Subconscious: persistent agent state, store activity queries

- Agent state (outputs) persists across runs in subconscious-state.json,
  loaded on startup, saved after each run completes
- Merge semantics: each run's outputs accumulate into persistent_state
  rather than replacing
- Walked keys restored from surface agent state on load
- Store::recent_by_provenance() queries nodes by agent provenance for
  the store activity view
- Switch outputs from HashMap to BTreeMap for stable display ordering

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

src/hippocampus/store/ops.rs

@@ -38,6 +38,17 @@ impl Store {
         Ok(())
     }
 
+    /// Recent nodes by provenance, sorted newest-first. Returns (key, timestamp).
+    pub fn recent_by_provenance(&self, provenance: &str, limit: usize) -> Vec<(String, i64)> {
+        let mut nodes: Vec<_> = self.nodes.values()
+            .filter(|n| !n.deleted && n.provenance == provenance)
+            .map(|n| (n.key.clone(), n.timestamp))
+            .collect();
+        nodes.sort_by(|a, b| b.1.cmp(&a.1));
+        nodes.truncate(limit);
+        nodes
+    }
+
     /// Upsert a node: update if exists (and content changed), create if not.
     /// Returns: "created", "updated", or "unchanged".
     ///