add updated_at to observations + observation review in curate flow
step 1: compact.py gets updated_at in schema and likes writes.
step 2: curate.py gains observation-specific tools (list_users,
list_user_observations, deprecate_observation, update_observation)
and a second agent phase that reviews private observations for
contradictions, staleness, and near-duplicates.
also fixes ruff errors: ambiguous var name, unused exception var,
unused logger import.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>