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/agent/oneshot.rs

@@ -55,7 +55,7 @@ pub struct AutoAgent {
     priority: i32,
     /// Named outputs from the agent's output() tool calls.
     /// Collected per-run, read by Mind after completion.
-    pub outputs: std::collections::HashMap<String, String>,
+    pub outputs: std::collections::BTreeMap<String, String>,
     // Observable status
     pub current_phase: String,
     pub turn: usize,
@@ -152,7 +152,7 @@ impl AutoAgent {
                 temperature, top_p: 0.95, top_k: 20,
             },
             priority,
-            outputs: std::collections::HashMap::new(),
+            outputs: std::collections::BTreeMap::new(),
             current_phase: String::new(),
             turn: 0,
         }