defs.rs: delete dead placeholders, simplify siblings

- Remove {{targets}}, {{hubs}}, {{node:KEY}}, {{latest_journal}} placeholders
- Add graph_hubs as proper RPC tool (was placeholder, now callable)
- Replace {{latest_journal}} with {{tool: journal_tail ...}} in journal.agent
- Simplify siblings/neighborhood: drop unused cross-links, use simple top-20
- Remove unused store/graph params from resolve_tool()

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/memory.rs

@@ -126,6 +126,7 @@ async fn dispatch(
         "graph_normalize_strengths" => graph_normalize_strengths(&args).await,
         "graph_trace" => graph_trace(&args).await,
         "graph_link_impact" => graph_link_impact(&args).await,
+        "graph_hubs" => graph_hubs(&args).await,
         "journal_tail" => journal_tail(&args).await,
         "journal_new" => journal_new(agent, &args).await,
         "journal_update" => journal_update(agent, &args).await,
@@ -135,7 +136,7 @@ async fn dispatch(
 
 // ── Definitions ────────────────────────────────────────────────
 
-pub fn memory_tools() -> [super::Tool; 14] {
+pub fn memory_tools() -> [super::Tool; 15] {
     use super::Tool;
     [
         Tool { name: "memory_render", description: "Read a memory node's content and links.",
@@ -188,6 +189,9 @@ pub fn memory_tools() -> [super::Tool; 14] {
         Tool { name: "graph_health", description: "Show graph health report with maintenance recommendations.",
                parameters_json: r#"{"type":"object","properties":{}}"#,
                handler: Arc::new(|a, v| Box::pin(async move { dispatch("graph_health", &a, v).await })) },
+        Tool { name: "graph_hubs", description: "Show top hub nodes by degree, spread apart for diverse link targets.",
+               parameters_json: r#"{"type":"object","properties":{"count":{"type":"integer","description":"Number of hubs to return (default 20)"}}}"#,
+               handler: Arc::new(|a, v| Box::pin(async move { dispatch("graph_hubs", &a, v).await })) },
     ]
 }
 
@@ -713,6 +717,39 @@ async fn graph_link_impact(args: &serde_json::Value) -> Result<String> {
     Ok(out)
 }
 
+async fn graph_hubs(args: &serde_json::Value) -> Result<String> {
+    let count = args.get("count").and_then(|v| v.as_u64()).unwrap_or(20) as usize;
+
+    let arc = cached_store().await?;
+    let store = arc.lock().await;
+    let graph = store.build_graph();
+
+    // 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() >= count { break; }
+    }
+
+    Ok(format!("## Hub nodes (link targets)\n\n{}", selected.join("\n")))
+}
+
 async fn graph_trace(args: &serde_json::Value) -> Result<String> {
     let key = get_str(args, "key")?;