CREATE TABLE snapshots (
	id TEXT PRIMARY KEY,
	task_id TEXT NOT NULL REFERENCES tasks(id),
	agent_session_id TEXT NOT NULL REFERENCES agent_sessions(id),
	n INTEGER NOT NULL,
	commit_sha TEXT NOT NULL,
	tree_sha TEXT NOT NULL,
	event_seq INTEGER NOT NULL,
	trigger TEXT NOT NULL,
	created_at TEXT NOT NULL,
	UNIQUE (task_id, n)
);