Snapshot task worktrees to hidden refs as agents work

internal/store/store.go

@@ -179,6 +179,19 @@ CREATE TABLE push_subscriptions (
 	subscriber TEXT NOT NULL,
 	created_at TEXT NOT NULL
 );
+`}, {"add-snapshots", `
+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)
+);
 `}}
 
 type Store struct {