Bulk-delete agents

Starts an asynchronous job that deletes up to 100 agents. Set `force=true` to delete execution history for every target, matching the single-agent delete endpoint; `force` must be a JSON boolean, and the strings `"true"` and `"false"` are rejected with 422 rather than coerced. A target with execution history and `force=false` fails with `execution_history_exists`. If the job itself fails, the target it stopped on carries `delete_failed` and every target behind it carries `not_attempted`, which means it was left untouched and is safe to resubmit. Authorization is captured when the request is accepted, not when the job runs: a role or organization granted after acceptance does not widen what the job may delete.