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/harness.ts

@@ -8,19 +8,26 @@ import AgentRegistry from '@deepseek-ai/dsh-agent'
 import AgentLoop, { ReactLoopAgent } from '@deepseek-ai/dsh-agent-loop'
 import { LocalBashExecutor } from '@deepseek-ai/dsh-bash-local'
 import * as ToolBash from '@deepseek-ai/dsh-tool-bash'
+import * as ToolTodo from '@deepseek-ai/dsh-tool-todo'
 import * as LlmDeepSeek from '@deepseek-ai/dsh-llm-deepseek'
 import SessionPersistenceJsonl from '@deepseek-ai/dsh-session-persistence-jsonl'
 
 /**
  * Shared harness for the coding-agent e2e suites: the full plugin stack
- * with the real DeepSeek adapter and the real bash tool. Lives outside the
- * *.e2e.ts pattern so importing it never re-registers another file's tests.
+ * with the real DeepSeek adapter and the real bash + todo_write tools. Lives
+ * outside the *.e2e.ts pattern so importing it never re-registers another
+ * file's tests.
  */
 
 export const SYSTEM_PROMPT = 'You are a coding agent. Your only tool is bash; '
   + 'do file operations with cat/grep/heredocs, check [exit code: N] markers, '
   + 'and report results briefly.'
 
+/** System prompt for the todo_write e2e: nudges the model to plan with the tool. */
+export const TODO_SYSTEM_PROMPT = 'You are a coding agent. For multi-step work, '
+  + 'use the todo_write tool to track a task list: send the WHOLE list each call, '
+  + 'keep exactly one task in_progress, and mark a task completed as soon as it is done.'
+
 export async function codingHarness(workdir: string, persistenceRoot?: string): Promise<Context> {
   const ctx = new Context()
   await ctx.plugin(LlmService)
@@ -32,6 +39,7 @@ export async function codingHarness(workdir: string, persistenceRoot?: string):
   await ctx.plugin(LlmDeepSeek, { models: ['deepseek-v4-flash'] })
   await ctx.plugin(LocalBashExecutor, { cwd: workdir, timeoutMs: 30_000 })
   await ctx.plugin(ToolBash)
+  await ctx.plugin(ToolTodo)
   // Durable JSONL persistence is opt-in: only the resume e2e needs it, and the
   // other suites stay file-free. Loaded last so a resume's deferred
   // `ctx.inject(['sessionPersistence'])` resolves once this is present.