Add an RCS agent

**Call this first.** It returns what a channel, country and sender type demand before you can send there — and `setup[]`, the calls that satisfy those demands, in order, each with a body you can copy and send as it stands. You never have to track forty countries' rules yourself. ## Channel `channel` is `sms`, `whatsapp` or `rcs`, and **defaults to `sms`** — so a caller that omits it gets exactly what this endpoint has always returned. `sms` is the only one answered today; the other two are `501` until their requirements are declared. They are refused rather than answered with an empty requirement set, because empty would say the channel demands nothing, and both demand plenty — WhatsApp a business verification and a display name review, RCS a carrier approval. Saying "not yet" is the true answer and the same one `POST /v3/sender-profiles` gives for `channels.whatsapp`. `type` is an SMS sender type, and SMS is the only channel whose senders have one. Requirements vary enormously. US `TEN_DLC` registers a brand and a campaign with The Campaign Registry and asks for a substantial set of values. An alphanumeric sender in one country pre-registers with supporting documents; in another it needs nothing at all. This is the authoritative answer for whichever market you are asking about, and it is the same declaration used to validate the calls themselves — so it can never drift from what is actually enforced. ## `setup[]` — what to do **One step.** `POST /v3/channels/sms` adds the market and records everything it registers with — the identity in `compliance.brand`, the programme in `compliance.campaign`. There were three, in a load-bearing order, while the identity and the campaign were resources of their own; they are members of this call's body now, so there is nothing to sequence. Correct any of it afterwards with `PATCH /v3/channels/sms/{country}/{type}`, which takes the same `compliance` object one key at a time. Each step's `body` has every key that market wants already present, with the values left **blank**. Blank rather than filled with placeholders, because the body is meant to be sent: a null value counts as not supplied, so posting a step verbatim comes back naming every blank you left. `attachments[]` lists files to send as multipart on the channel call, each under the field name given — a file cannot be a JSON value, so documents are never listed in `compliance[]`. Every step is a call for whoever holds the key. To set a **sender profile** up instead, send the same requests with the `x-profile-id` header naming it; there is no separate path. ## `requirements[]` — what each value means Each entry names a key for the `compliance` array, its value type, and whether it is required. `required_when` marks values needed only in certain cases — evaluate it against your own payload, e.g. `brand.entity_type == PUBLIC_PROFIT`. `options` lists accepted values where the market constrains them, and `label` is suitable for rendering a form field. Values Sent supplies on your behalf are never listed: correlation ids, environment flags, carrier elections, and booleans derivable from what you already send. ## When nothing is required `required` is `false` and `requirements` is empty, and the single `setup` step carries no `compliance` array at all. A `sender_id_pattern` may still be present — many markets constrain the sender string without registering anything.