Limit concurrent checks per scope

The new scopes.capacities map declares a per-scope checks budget on top of merge_queue.max_parallel_checks. A pull request is checked only when the global ceiling and every scope it touches have room, so noisy or slow scopes don’t crowd out the rest of the train. The same syntax works whether scopes come from file globs (source: files) or the manual scopes API (source: manual), and scopes you don’t list stay unlimited (only the global ceiling applies).