Wire Axon /axon/message to NeuronRuntime dispatch

- axon.rs: replace stub AxonHandler with live NeuronRuntime dispatch;
  tool calls now route through Engram VM when modules are loaded
- state.rs: load .el modules from NEURON_LANG_DIR (default ~/.neuron/lang);
  falls back to empty dir gracefully when not present
- Cargo.toml: add engram-core dependency for EngramDb

Cargo.toml

@@ -6,6 +6,9 @@ members = [
     "crates/neuron-api",
     "crates/neuron-protocol",
     "crates/axon-events",
+    "crates/neuron-runtime",
+    "crates/neuron-migrate",
+    "crates/neuron-cli",
 ]
 
 [workspace.dependencies]