feat: 13-language AST support and token benchmark

Java, C, C++, Ruby, C#, Kotlin, Scala, PHP via tree-sitter (13 total)
benchmark.py measures BFS subgraph tokens vs corpus tokens
5 skill bug fixes (cohesion crash, dead step, missing MCP tool)

tests/fixtures/sample.java

@@ -0,0 +1,32 @@
+import java.util.List;
+import java.util.ArrayList;
+
+public class DataProcessor {
+    private List<String> items;
+
+    public DataProcessor() {
+        this.items = new ArrayList<>();
+    }
+
+    public void addItem(String item) {
+        items.add(item);
+    }
+
+    public List<String> process() {
+        return validate(items);
+    }
+
+    private List<String> validate(List<String> data) {
+        List<String> result = new ArrayList<>();
+        for (String s : data) {
+            if (s != null && !s.isEmpty()) {
+                result.add(s.trim());
+            }
+        }
+        return result;
+    }
+}
+
+interface Processor {
+    List<String> process();
+}