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/mock-llm.ts

@@ -0,0 +1,59 @@
+import type { Context } from 'cordis'
+import type { GenerateOptions, StreamChunk } from '@deepseek-ai/dsh-llm'
+import { LlmAdapter } from '@deepseek-ai/dsh-llm'
+
+/**
+ * Demo adapter for the `mock-echo` model.
+ *
+ * Behavior: if the last user text starts with "echo ", it calls the `echo`
+ * tool with the rest of the line (exercising the tool round-trip), otherwise
+ * it streams a canned reply quoting the input.
+ */
+class MockEchoAdapter extends LlmAdapter {
+  async * stream(options: GenerateOptions): AsyncIterable<StreamChunk> {
+    const lastUserText = [...options.messages].reverse()
+      .filter(message => message.role === 'user')
+      .flatMap(message => message.content)
+      .filter(block => block.type === 'text')
+      .map(block => block.text)
+      .find(text => !text.startsWith('<')) ?? ''
+
+    const hasToolResult = options.messages.at(-1)?.content.some(block => block.type === 'tool-result')
+
+    if (lastUserText.startsWith('echo ') && !hasToolResult) {
+      const payload = lastUserText.slice(5)
+      const args = JSON.stringify({ text: payload })
+      yield { type: 'block-start', index: 0, blockType: 'text' }
+      for (const char of 'Let me echo that for you.') {
+        yield { type: 'text-delta', index: 0, text: char }
+        await new Promise(resolve => setTimeout(resolve, 2))
+      }
+      yield { type: 'block-end', index: 0, block: { type: 'text', text: 'Let me echo that for you.' } }
+      yield { type: 'block-start', index: 1, blockType: 'tool-call' }
+      yield { type: 'tool-call-delta', index: 1, id: 'call-echo', name: 'echo', argumentsDelta: args }
+      yield { type: 'block-end', index: 1, block: { type: 'tool-call', id: 'call-echo', name: 'echo', arguments: args } }
+      yield { type: 'usage', usage: { inputTokens: 20, outputTokens: 10 } }
+      yield { type: 'finish', reason: { kind: 'tool-calls' } }
+      return
+    }
+
+    const reply = hasToolResult
+      ? 'The echo tool has spoken.'
+      : `You said: "${lastUserText}". Try "echo <something>" to see a tool call.`
+    yield { type: 'block-start', index: 0, blockType: 'text' }
+    for (const char of reply) {
+      yield { type: 'text-delta', index: 0, text: char }
+      await new Promise(resolve => setTimeout(resolve, 2))
+    }
+    yield { type: 'block-end', index: 0, block: { type: 'text', text: reply } }
+    yield { type: 'usage', usage: { inputTokens: 20, outputTokens: reply.length } }
+    yield { type: 'finish', reason: { kind: 'stop' } }
+  }
+}
+
+export const name = 'mock-llm'
+export const inject = ['llm']
+
+export function apply(ctx: Context) {
+  ctx.llm.registerAdapter(['mock-echo'], new MockEchoAdapter())
+}