Create a playbook definition

Creates a new organization-level playbook definition; it starts in `draft` status. Authorization: organization administrators only — other callers are rejected with 403. Body: `name` (required, 1-200 chars); optional `description` (<=5000 chars); optional `spaceTypes` (array of space-type slugs the playbook applies to — omit/empty means all space types); optional `tasks` (<=30 items, each `{ id (1-64), title (1-200), instructions?, dependsOn? (ids of tasks that must complete first) }`); optional `actions` (<=20 items, each `{ installedActionId, preApproval?, postApproval?, dependsOn? (task ids only — an action may not depend on another action) }` referencing installed actions, with per-action human-approval gates). Response 201: `{ data: Playbook }`.