PR #27371 introduced a per-user-peer resolver in HonchoSessionManager,
but the resolved runtime identity is frozen into the manager at first-
message init. When the gateway session_key intentionally omits the
participant ID (the default for threads via thread_sessions_per_user=
False), a cached AIAgent created by user A is reused for user B's
messages, attributing B's writes to A's resolved Honcho peer and
breaking #27371's per-user-peer contract.
Fix by including user_id and user_id_alt in _agent_config_signature so
the cache key distinguishes participants in shared threads. Each user
in a shared thread now triggers a fresh AIAgent build (trading prompt-
cache warmth for memory-attribution correctness — the right tradeoff
for an external-memory backend where misattribution is unrecoverable).
The default-None case keeps the signature byte-identical to pre-fix
behavior so this change doesn't invalidate in-flight caches on deploy.
c03960decd