#84 Add per-account session and identity persistence

Parameterize session.rs: load_client(did), persist_session(did, session) use account dir. Add resolve_context(as_flag: Option<&str>) -> CommandContext { did, pds_url }. Identity: load_identity(did)/save_identity(did)/ensure_identity(did, rng) paths switch to per-account dirs. Depends on #50 for account_dir().