Enha: add agent to chat history; gen-regen diff

storage/migrations/001_init.up.sql

@@ -2,6 +2,7 @@ CREATE TABLE IF NOT EXISTS chats (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     name TEXT NOT NULL,
     msgs TEXT NOT NULL,
+    agent TEXT NOT NULL DEFAULT 'assistant',
     created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
 );