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/acp-agent/tests/acp.snapshot.ts

@@ -52,6 +52,7 @@ const SCENARIOS: Scenario[] = [
   { name: 'reject-extra-dirs', hasModelTurn: false, recorded: false },
   { name: 'text-turn', hasModelTurn: true, recorded: true },
   { name: 'tool-call-turn', hasModelTurn: true, recorded: true },
+  { name: 'todo-plan', hasModelTurn: true, recorded: true },
   { name: 'workspace-edit', hasModelTurn: true, recorded: true },
   { name: 'multi-turn', hasModelTurn: true, recorded: true },
   { name: 'error-finish', hasModelTurn: true, recorded: false },