Start provider sign-in

Starts the provider's interactive flow. Follow next_action and poll until terminal. Codex and Radius default to device-code authentication; other interactive flows may require pasting a callback URL from the user's browser. Copilot supports github.com accounts only; answer its Enterprise-domain prompt with an empty string. Starting again cancels only this user's previous login. Requires a workspace-bound user OAuth grant with org:write and owner/admin membership. API keys and browser sessions are not accepted. Login challenges are private to the initiating user. Sessions are persisted for recovery across API instances and restarts until expires_at. An uncertain token exchange is not replayed: an explicit restart-required failure means start a fresh login, retaining account_key. After expiry or a 404, start a new login. Provider device codes may expire sooner. Never share login URLs or codes.

_{Scope: org:write}