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-memory/src/schema.rs

@@ -1,7 +1,7 @@
 //! Database schema definitions
 
 /// Current schema version
-pub const SCHEMA_VERSION: i32 = 1;
+pub const SCHEMA_VERSION: i32 = 2;
 
 /// Schema creation SQL
 pub const CREATE_SCHEMA: &str = r#"
@@ -87,3 +87,10 @@ CREATE INDEX IF NOT EXISTS idx_facts_agent ON facts(agent_id);
 CREATE INDEX IF NOT EXISTS idx_facts_category ON facts(agent_id, category);
 CREATE INDEX IF NOT EXISTS idx_facts_confidence ON facts(agent_id, confidence DESC);
 "#;
+
+/// Incremental migrations (safe to run repeatedly via ALTER … ADD COLUMN + IF NOT EXISTS pattern)
+pub const MIGRATIONS: &[&str] = &[
+    // v1→v2: persist runtime state and message count
+    "ALTER TABLE agents ADD COLUMN state TEXT NOT NULL DEFAULT 'running'",
+    "ALTER TABLE agents ADD COLUMN message_count INTEGER NOT NULL DEFAULT 0",
+];