Start an action run in a space

Starts a new action run scoped to the space in the path. Requires space:contribute on that space (denied → 403). The space is taken from the path — a spaceId in the body never scopes the run. Body: actionSlug (installed action to run, required, non-empty), fileIds (1–100 file IDs from the space to feed the run, each non-empty), optional params (free-form object passed to the action), optional config (inline action config, deep-merged over the install’s saved config — inline wins — and snapshotted onto the run, so an action can be run without saving its config first). Returns 201 { data: ActionRun }; the run starts in status running — poll getRun for awaiting_input / done / failed plus its outputs and artifacts.