fix(checkpoint): preserve main data in mergeShards

Fix mergeShards to preserve existing main checkpoint data instead
of starting from empty state. This maintains totalPlanned and
gitCommit metadata when merging parallel execution shards.

Author: Mearman

src/experiments/framework/executor/checkpoint-manager.ts

@@ -437,13 +437,16 @@ export class CheckpointManager {
 	 * @returns The merged checkpoint data
 	 */
 	async mergeShards(shardPaths: string[]): Promise<CheckpointData> {
+		// Start with main checkpoint data, or empty if no checkpoint exists yet
+		const mainData = this.data;
 		const merged: CheckpointData = {
-			configHash: "",
-			createdAt: new Date().toISOString(),
+			configHash: mainData?.configHash ?? "",
+			createdAt: mainData?.createdAt ?? new Date().toISOString(),
 			updatedAt: new Date().toISOString(),
-			completedRunIds: [],
-			results: {},
-			totalPlanned: 0,
+			completedRunIds: [...(mainData?.completedRunIds ?? [])],
+			results: { ...(mainData?.results ?? {}) },
+			totalPlanned: mainData?.totalPlanned ?? 0,
+			gitCommit: mainData?.gitCommit,
 		};
 
 		// Load and merge each shard