test(todo): add the todo-plan ACP snapshot scenario and a with-key e2e

Record the `todo-plan` snapshot scenario: a real prompt drives the model to call
todo_write, and the golden captures the resulting `plan` sessionUpdate (three
entries, priority synthesized as medium, status 1:1) plus the persisted
todo/write event. Registered in SCENARIOS; replays deterministically keyless.

Add a with-key coding-agent e2e that verifies the WORLD — a real model call to
todo_write lands a todo/write event whose snapshot is a valid, one-in-progress
list — not the agent's self-report. Wire tool-todo into the e2e harness.

examples/coding-agent/tests/todo-write.e2e.ts

@@ -0,0 +1,55 @@
+import { afterEach, describe, expect, it } from 'vitest'
+import type { Context } from 'cordis'
+import { AgentId } from '@deepseek-ai/dsh-agent'
+import type { TodoItem } from '@deepseek-ai/dsh-session'
+import { codingHarness, TODO_SYSTEM_PROMPT, waitForIdle } from './harness.ts'
+
+/**
+ * A REAL model drives the REAL todo_write tool: verify the WORLD (the session
+ * log gains a todo/write event whose snapshot the model actually produced), not
+ * the agent's self-report. Key-gated (see vitest.e2e.config.ts).
+ */
+
+let ctx: Context | undefined
+
+afterEach(async () => {
+  await ctx?.fiber.dispose()
+  ctx = undefined
+})
+
+describe.skipIf(!process.env.DEEPSEEK_API_KEY)('todo_write: real model records a plan', () => {
+  it('appends a todo/write event with the model-produced task list', async () => {
+    ctx = await codingHarness(process.cwd())
+    const agent = ctx.agentLoop.create(AgentId('e2e-todo'), {
+      model: 'deepseek-v4-flash',
+      systemPrompt: TODO_SYSTEM_PROMPT,
+    })
+
+    agent.send([{ type: 'text', text:
+      'Use the todo_write tool to record a plan of exactly two steps: first '
+      + '"inspect the failing test" (in_progress), then "apply the fix" (pending). '
+      + 'Send both in one todo_write call, then reply with the single word DONE.' }])
+    await waitForIdle(ctx, agent)
+
+    const events = [...agent.session.events]
+
+    // The model actually called the tool.
+    const calls = events.filter(event => event.type === 'tool/call')
+    expect(calls.some(event => event.data.name === 'todo_write')).toBe(true)
+
+    // And the tool wrote a todo/write event to the log — verify the WORLD.
+    const todoEvents = events.filter(event => event.type === 'todo/write')
+    expect(todoEvents.length).toBeGreaterThan(0)
+
+    const todos = (todoEvents.at(-1)!).data.todos
+    expect(todos.length).toBeGreaterThanOrEqual(2)
+    // Every entry has a non-empty content and a valid status…
+    const valid: TodoItem['status'][] = ['pending', 'in_progress', 'completed']
+    for (const todo of todos) {
+      expect(todo.content.trim().length).toBeGreaterThan(0)
+      expect(valid).toContain(todo.status)
+    }
+    // …and the one-in-progress invariant the tool enforces held.
+    expect(todos.filter(t => t.status === 'in_progress').length).toBeLessThanOrEqual(1)
+  }, 120_000)
+})