agents: {{HUBS}} placeholder for top 20 spread-apart hub nodes

New placeholder resolves to the 20 highest-degree nodes, skipping
neighbors of already-selected hubs so the list covers different
regions of the graph. Gives agents a starting point for linking
new content to the right places.

Added to observation.agent prompt.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/agents/observation.agent

@@ -65,6 +65,8 @@ If there's nothing worth extracting, just say so.
 - **Specific over vague.**
 - **Don't force it.** "Nothing new here" is valid output.
 
+{{HUBS}}
+
 ## Conversation fragments to mine
 
 {{CONVERSATIONS}}

poc-memory/src/agents/defs.rs

@@ -271,6 +271,34 @@ fn resolve(
             })
         }
 
+        "hubs" => {
+            // Top hub nodes by degree, spread apart (skip neighbors of already-selected hubs)
+            let mut hubs: Vec<(String, usize)> = store.nodes.iter()
+                .filter(|(k, n)| !n.deleted && !k.starts_with('_'))
+                .map(|(k, _)| {
+                    let degree = graph.neighbors(k).len();
+                    (k.clone(), degree)
+                })
+                .collect();
+            hubs.sort_by(|a, b| b.1.cmp(&a.1));
+
+            let mut selected = Vec::new();
+            let mut seen: std::collections::HashSet<String> = std::collections::HashSet::new();
+            for (key, degree) in &hubs {
+                if seen.contains(key) { continue; }
+                selected.push(format!("  - {} (degree {})", key, degree));
+                // Mark neighbors as seen so we pick far-apart hubs
+                for (nbr, _) in graph.neighbors(key) {
+                    seen.insert(nbr.clone());
+                }
+                seen.insert(key.clone());
+                if selected.len() >= 20 { break; }
+            }
+
+            let text = format!("## Hub nodes (link targets)\n\n{}", selected.join("\n"));
+            Some(Resolved { text, keys: vec![] })
+        }
+
         // node:KEY — inline a node's content by key
         other if other.starts_with("node:") => {
             let key = &other[5..];