agents: model dispatch from .agent file, add generalize agent WIP

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.

poc-memory/agents/naming.agent

@@ -1,4 +1,4 @@
-{"agent":"naming","query":"","model":"haiku","schedule":""}
+{"agent":"naming","query":"","model":"sonnet","schedule":""}
 # Naming Agent — Node Key Resolution
 
 You are given a proposed new node (key + content) and a list of existing