fix(ci): repoint the demo smoke test at the dsh-stdio-agent bin

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).

.github/workflows/ci.yml

@@ -81,7 +81,7 @@ jobs:
       - name: Demo smoke test
         run: |
           set -euo pipefail
-          out=$(printf 'echo ci smoke\n' | timeout 60 node --expose-internals --import tsx examples/echo-agent/start.ts 2>&1)
+          out=$(printf 'echo ci smoke\n' | timeout 60 pnpm run demo:echo 2>&1)
           echo "$out"
           echo "$out" | grep -q '\[tool call\] echo({"text":"ci smoke"})'
           echo "$out" | grep -q '\[tool result\] ECHO: CI SMOKE'