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/user/subconscious.rs

@@ -241,14 +241,18 @@ impl SubconsciousScreen {
                 }
             }
 
-            if !snap.walked.is_empty() {
-                lines.push(Line::raw(""));
-                lines.push(Line::styled(
-                    format!("  walked ({}):", snap.walked.len()),
-                    Style::default().fg(Color::Cyan),
-                ));
-                for key in &snap.walked {
-                    lines.push(Line::styled(format!("    {}", key), dim));
+            if let Some(walked_str) = snap.state.get("walked") {
+                let walked: Vec<&str> = walked_str.lines()
+                    .map(|l| l.trim()).filter(|l| !l.is_empty()).collect();
+                if !walked.is_empty() {
+                    lines.push(Line::raw(""));
+                    lines.push(Line::styled(
+                        format!("  walked ({}):", walked.len()),
+                        Style::default().fg(Color::Cyan),
+                    ));
+                    for key in &walked {
+                        lines.push(Line::styled(format!("    {}", key), dim));
+                    }
                 }
             }
         }