Add runnable echo-agent example

cordis.yml-wired demo proving the full stack end to end: mock-echo
LlmAdapter (streams text; calls the echo tool on "echo <text>"),
echo tool, stdio chat UI plugin (consumes only the agent/* taxonomy),
and a JSONL persistence plugin demonstrating the write-behind +
session/flush checkpoint pattern. Runs unbuilt via tsx with loader,
include, and HMR live-reload all active (yarn demo).

examples/echo-agent/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "echo-agent-example",
+  "private": true,
+  "version": "0.0.1",
+  "type": "module",
+  "description": "Runnable demo: stdin chat with a scripted mock model + echo tool"
+}