Make call_model pub(crate) so run_one_agent reads the model from the
.agent definition instead of hardcoding sonnet. Naming agent upgraded
from haiku to sonnet.
Add generalize agent: finds the largest prefix-grouped cluster of
nodes that hasn't been visited recently, wired into the agent cycle
between extractor and connector at depth 3. New "clusters" resolver
in defs.rs does prefix-based grouping with provenance filtering.
