fix: create log directory before running agent container (#168)

[kaixinbaba]

Fixes #168

Problem

The run-job.yml workflow fails because the Docker container expects a logs/{JOB_ID} directory to exist on the host volume, but the workflow never creates it. This causes:
Error: ENOENT: no such file or directory, open 'logs/.../stdout.log'

Fix

Add a "Prepare log directory" step before the Docker run step that creates the required directory:

JOB_ID="${GITHUB_REF_NAME#job/}"
mkdir -p "logs/${JOB_ID}"

This ensures the directory exists before the agent container tries to write logs to it.