Adds lifecycle mutations so a stack can shrink or end without manual repair. - `POST /api/v3/repos/:owner/:repo/stacks/:stack_number/unstack` removes the top layer, returns the pull request to a standalone review, and dissolves the stack when the last layer leaves. - `POST .../stacks/:stack_number/dissolve` releases every layer at once and closes the stack. - Both mutations replay on a repeated `Idempotency-Key`, refuse to run while a durable operation is active, and record `pull_request.unstacked` and `pull_request_stack.dissolved` events. - A stack now holds at most 100 active layers, enforced on create and append. Closes #113