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/stdio-chat.ts

@@ -0,0 +1,60 @@
+import { createInterface } from 'node:readline'
+import type { Context } from 'cordis'
+import type {} from '@deepseek-ai/dsh-agent'
+
+export const name = 'stdio-chat'
+export const inject = ['agents']
+
+/**
+ * Minimal UI plugin: reads lines from stdin → agent.send(); renders the
+ * agent's stream chunks and tool activity to stdout. Demonstrates that a UI
+ * is "just a plugin" — it only consumes the agent/* event taxonomy.
+ */
+export function apply(ctx: Context) {
+  ctx.on('agent/stream-chunk', (agent, _turn, _step, chunk) => {
+    if (chunk.type === 'text-delta') process.stdout.write(chunk.text)
+  })
+
+  ctx.on('agent/turn-start', (agent, turn) => {
+    process.stdout.write(`\n[${agent.id} turn ${turn}] `)
+  })
+
+  ctx.on('agent/turn-end', () => {
+    process.stdout.write('\n> ')
+  })
+
+  ctx.on('session/event', (_session, event) => {
+    if (event.type === 'tool/call') {
+      const { name: toolName, arguments: args } = event.data as any
+      process.stdout.write(`\n  [tool call] ${toolName}(${args})`)
+    } else if (event.type === 'tool/result') {
+      const { content } = event.data as any
+      const text = content.filter((b: any) => b.type === 'text').map((b: any) => b.text).join('')
+      process.stdout.write(`\n  [tool result] ${text}\n  `)
+    }
+  })
+
+  ctx.effect(() => {
+    const reader = createInterface({ input: process.stdin })
+    reader.on('line', (line) => {
+      const text = line.trim()
+      if (!text) return
+      const agent = ctx.agents.get('main')
+      if (!agent) {
+        console.error('agent "main" is not running')
+        return
+      }
+      if (agent.status === 'running') {
+        agent.steer([{ type: 'text', text }])
+      } else {
+        agent.send([{ type: 'text', text }])
+      }
+    })
+    reader.on('close', () => {
+      // allow the process to exit when stdin ends (piped input)
+      setTimeout(() => process.exit(0), 200)
+    })
+    process.stdout.write('echo-agent ready. Type a message ("echo <text>" triggers the tool).\n> ')
+    return () => reader.close()
+  }, 'stdio-chat')
+}