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/start.ts

@@ -0,0 +1,16 @@
+import { pathToFileURL } from 'node:url'
+import { Context } from 'cordis'
+import Loader from '@cordisjs/plugin-loader'
+
+// Boot a Cordis app from this example's cordis.yml — the same shape as the
+// upstream `cordis` bin, pinned to this directory.
+const ctx = new Context()
+ctx.baseUrl = pathToFileURL(import.meta.dirname).href + '/'
+
+await ctx.plugin(Loader)
+await ctx.loader.create({
+  name: '@cordisjs/plugin-include',
+  config: {
+    path: './cordis.yml',
+  },
+})