[codex] capture stdin before runner compile

[DorianZheng]

Summary

This change fixes a BoxLite startup regression where the agent container could fail with Failed to parse input: Unexpected end of JSON input before the runner started.

What changed

• capture stdin into /tmp/input.json before any entrypoint startup work
• keep the runner recompilation step after stdin has already been preserved
• add a regression test that asserts the Dockerfile entrypoint captures stdin before invoking npx tsc

Root cause

The entrypoint was recompiling the mounted runner source before reading stdin. In the BoxLite execution path, that startup ordering could leave the runner reading an empty or truncated input stream, which then failed at JSON.parse(stdinData).

Impact

This makes container startup deterministic for BoxLite-backed runs and prevents the main failure mode reported in the box log.

Validation

• npm test -- src/container-image.test.ts src/container-runner.test.ts