Report the current session's identity. Read-only, no sign-in required: an
anonymous session gets `{authenticated: false}` with a hint (not an error),
a signed-in one gets `{authenticated: true}` plus the GitHub-rooted id, login
and tariff. Call it to confirm who you are before `register_identity` /
`store_memory`; an anonymous caller must sign in (GitHub OAuth) first.
Connector