context: heal pre-refactor image logs with token_count=0

Recompute image token counts from persisted dimensions when loading
old logs that stored count=0 (server-authoritative count was applied
after AppendImage before client-side pad expansion).

graph: cache neighbor sets for clustering coefficient

Pre-compute neighbor HashSets so the O(deg^2) triangle-counting
inner loop doesn't re-allocate on every (i,j) pair. avg_clustering_
coefficient() now builds the cache once instead of O(N*deg) times.

src/agent/context.rs

@@ -125,7 +125,19 @@ impl<'de> Deserialize<'de> for NodeLeaf {
             body: NodeBody,
             timestamp: DateTime<Utc>,
         }
-        let raw = Raw::deserialize(deserializer)?;
+        let mut raw = Raw::deserialize(deserializer)?;
+        // Heal pre-refactor logs: Image leaves used to be deserialized
+        // with token_count=0 (server-authoritative count was applied
+        // after AppendImage). With pads now expanded client-side at
+        // construction, recompute from the persisted dimensions if
+        // the stored count is 0.
+        if let NodeBody::Image { orig_height, orig_width, token_count, .. }
+            = &mut raw.body
+        {
+            if *token_count == 0 {
+                *token_count = qwen3_image_token_count(*orig_height, *orig_width);
+            }
+        }
         let token_ids = raw.body.compute_token_ids();
         Ok(NodeLeaf { body: raw.body, token_ids, timestamp: raw.timestamp })
     }