graph: community isolation scoring + sort:isolation query

Add community_isolation() to Graph — computes per-community ratio of
internal vs total edge weight. 1.0 = fully isolated, 0.0 = all edges
external.

New query: sort:isolation — sorts nodes by their community's isolation
score, most isolated first. Useful for aiming organize agents at
poorly-integrated knowledge clusters.

New CLI: poc-memory graph communities [N] [--min-size M] — lists
communities sorted by isolation with member preview. Reveals islands
like the Shannon theory cluster (3 nodes, 100% isolated, 0 cross-edges)
and large agent-journal clusters (20-30 nodes, 95% isolated).

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

poc-memory/src/cli/graph.rs

@@ -656,3 +656,40 @@ pub fn cmd_interference(threshold: f32) -> Result<(), String> {
     Ok(())
 }
 
+/// Show communities sorted by isolation (most isolated first).
+/// Useful for finding poorly-integrated knowledge clusters that need
+/// organize agents aimed at them.
+pub fn cmd_communities(top_n: usize, min_size: usize) -> Result<(), String> {
+    let store = store::Store::load()?;
+    let g = store.build_graph();
+    let infos = g.community_info();
+
+    let total = infos.len();
+    let shown: Vec<_> = infos.into_iter()
+        .filter(|c| c.size >= min_size)
+        .take(top_n)
+        .collect();
+
+    println!("{} communities total ({} with size >= {})\n",
+        total, shown.len(), min_size);
+    println!("{:<6} {:>5} {:>7} {:>7}  members", "id", "size", "iso", "cross");
+    println!("{}", "-".repeat(70));
+
+    for c in &shown {
+        let preview: Vec<&str> = c.members.iter()
+            .take(5)
+            .map(|s| s.as_str())
+            .collect();
+        let more = if c.size > 5 {
+            format!(" +{}", c.size - 5)
+        } else {
+            String::new()
+        };
+        println!("{:<6} {:>5} {:>6.0}% {:>7}  {}{}",
+            c.id, c.size, c.isolation * 100.0, c.cross_edges,
+            preview.join(", "), more);
+    }
+
+    Ok(())
+}
+