The CI "Demo smoke test" step booted the deleted examples/echo-agent/start.ts
(removed when the boot glue moved into the dsh-stdio-agent bin), so CI failed on
node 24 + 26 while the local gates and e2e passed. Invoke `pnpm run demo:echo`
instead of hardcoding the boot path — that routes through the canonical demo
script, so the smoke can never drift from it again. The output assertions and
the .sessions/_no-cwd/main-session-*.jsonl artifact check are unchanged
(verified the new bin produces identical output).