Adds an asynchronous merge submission API for stacked pull requests. - `PUT /api/v3/repos/:owner/:repo/pulls/:pull_number/merge-async` accepts a merge for the layer's stack, returns `202` with a durable operation ID, and replays on a repeated `Idempotency-Key`. - `GET .../merge-async/:operation_id` reports `pending`, `merged`, or `failed` with the operation state and error. - A conflicting active operation returns `409` with the existing operation ID so a client can poll it instead of guessing. - The merge keeps the contiguous-prefix contract and restacks the remaining upper layers automatically after a partial merge. Closes #112