What problem does this feature solve?

When using test sharding (--shard) to split tests across multiple CI machines, each shard produces its own test report and coverage data independently. There is currently no built-in mechanism to merge the results from all shards into a single unified report and coverage summary.

This is a common CI workflow requirement — users split tests into N shards for parallelism (e.g., --shard 1/3, --shard 2/3, --shard 3/3), and need to combine the outputs afterward to get a complete picture of test results and coverage.

What does the proposed API look like?

Related context

• Test sharding is already supported via --shard <index>/<count> (see docs)
• Vitest provides --merge-reports to handle this workflow
• Referenced in #85 (comment)