feat(kernel): persist agent runtime state across restarts

- Schema: migrations now execute ALTER TABLE ADD COLUMN for state/message_count
- MemoryStore: add update_agent_runtime() and list_agents_with_runtime()
- Registry: add register_with_runtime() to accept persisted state/message_count
- Kernel boot: restore agents with their persisted state (not always Running)
- Kernel shutdown: persist all agent states/message_counts before terminating

Agents that were suspended stay suspended after restart. Message counts
survive restarts instead of resetting to 0.

crates/zclaw-kernel/src/registry.rs

@@ -30,6 +30,22 @@ impl AgentRegistry {
         self.created_at.insert(id, Utc::now());
     }
 
+    /// Register an agent with persisted runtime state
+    pub fn register_with_runtime(
+        &self,
+        config: AgentConfig,
+        state: AgentState,
+        message_count: u64,
+    ) {
+        let id = config.id;
+        self.agents.insert(id, config);
+        self.states.insert(id, state);
+        self.created_at.insert(id, Utc::now());
+        if message_count > 0 {
+            self.message_counts.insert(id, message_count);
+        }
+    }
+
     /// Unregister an agent
     pub fn unregister(&self, id: &AgentId) {
         self.agents.remove(id);