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/src/echo-tool.ts

@@ -0,0 +1,19 @@
+import type { Context } from 'cordis'
+
+export const name = 'echo-tool'
+export const inject = ['tools']
+
+export function apply(ctx: Context) {
+  ctx.tools.register({
+    name: 'echo',
+    description: 'Echo the given text back, uppercased.',
+    parameters: {
+      type: 'object',
+      properties: { text: { type: 'string' } },
+      required: ['text'],
+    },
+    async execute(args: any) {
+      return [{ type: 'text', text: `ECHO: ${String(args?.text ?? '').toUpperCase()}` }]
+    },
+  })
+}