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

crates/neuron-api/Cargo.toml

@@ -11,6 +11,8 @@ path = "src/main.rs"
 neuron-domain = { path = "../neuron-domain" }
 neuron-store = { path = "../neuron-store" }
 neuron-protocol = { path = "../neuron-protocol" }
+neuron-runtime = { path = "../neuron-runtime" }
+chrono = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 uuid = { workspace = true }