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/kernel/agents.rs

@@ -1,6 +1,6 @@
 //! Agent CRUD operations
 
-use zclaw_types::{AgentConfig, AgentId, AgentInfo, Event, Result};
+use zclaw_types::{AgentConfig, AgentId, AgentInfo, AgentState, Event, Result};
 
 #[cfg(feature = "multi-agent")]
 use std::sync::Arc;