Create an envelope

Creates a draft envelope with its participant and field graph. The document arrives by public URL, inline base64 content, chat-authored text rendered to PDF, a same-organization envelope document, an ordered documents array of two or more PDFs merged into one document, or deferred upload ({ upload: true } returns a short-lived signed uploadUrl; it is never re-issued after create). Returns immediately with the envelope, a review URL for the web review page, a human/agent-readable note describing the next step, and never blocks on field detection. Multi-document create is PDF-only until ATL-39 adds DOCX conversion, and deferred upload remains single-document. URL and inline documents enqueue field detection asynchronously after the document is stored (the envelope moves draft → detecting → review); deferred-upload envelopes skip detection and remain sendable with manually placed fields. Supports the Idempotency-Key header: a replayed completed key returns the envelope's current graph with the original status code and Idempotency-Replayed: true (never re-executing); an in-flight duplicate is rejected 409; reusing a key with a different request is rejected 422. A replayed create re-mints the review URL but never the one-shot upload URL.