Adds observationMode config field to HonchoClientConfig:
- 'unified' (default): user peer self-observations, all agents share one pool
- 'directional': AI peer observes user, each agent keeps its own view
Changes:
- client.py: observation_mode field, _normalize_observation_mode(), config resolution
- session.py: add_peers respects mode (peer observation flags), dialectic_query
routes through correct peer, create_conclusion uses correct observer