Wraps the createSession + Authorization: Bearer flow so callers don't
need to assemble curl by hand. Reads BSKY_IDENTIFIER / BSKY_PASSWORD /
BSKY_PDS / REMANSO_API from env (or matching flags). Defaults to
bsky.social so non-bsky-hosted accounts must set BSKY_PDS explicitly,
e.g. BSKY_PDS=https://eurosky.social.
