Change a namespace's settings

Only the fields you send are changed; anything omitted is left as it is, so {} changes nothing. Sending instructions: null, or an empty or whitespace-only string, clears them and returns summaries to the neutral default.

Instructions are capped at 2000 characters. A longer note is refused rather than truncated, because a note cut mid-sentence is a worse steer than none. A change reaches each summary the next time that summary is regenerated, not immediately.