Add level descriptions

src/levels/l06.md

@@ -0,0 +1,11 @@
+Sixth level introduces anchors. Two rooms share the same trap — define it once and alias it.
+
+trap: &spike_pit
+  type: pit
+  depth: 20
+  spikes: true
+rooms:
+  north: *spike_pit
+  south: *spike_pit
+
+Randomize the trap properties and which rooms reuse it