Add a GitHub organisation through your own GitHub login

Body { installationId }, taken from /github/reachable. Allowed while other workspaces use the installation and while this workspace already has connections. Re-checks with GitHub, using the caller’s own token, that they can reach it; one they cannot gets the same 404 as one that does not exist. Recorded as a person’s reach and never as company property — in particular an API key does not read through it, and every deploy is still checked against the person’s own GitHub access — even when the caller is an owner. Idempotent. Same policy as members connecting an account: an owner always, a member while the workspace allows it. A signed-in person; an API key is refused.