Replace the avatar's knowledge files

Replaces the whole knowledge set: repeat the files part for each document (pdf, docx, doc, txt, md, html, csv <= 50 MB each; up to 20 files). Send with no files to clear it. descriptions is an optional JSON object mapping a filename to a short description. Or, on an instance that keeps uploads (where POST /v1/uploads answers), application/json: {"files": ["<https URL | upload:<uploadId>>", ...], "descriptions": {...}}, the files by URL or by knowledge uploads (an empty list clears the set). A URL's file is named by its last path segment, an upload's by the fileName it was created with; descriptions uses those names. The uploads are consumed when the replacement succeeds. Re-runs only the build stages that depend on this part; poll GET /v1/avatars/{avatarId} for progress. Requires avatars:write. The knowledge.facts text and the knowledge.questionnaire answers keep their own files: replacing the files leaves them (change them with PATCH). Knowledge is not used in v2 conversations yet.