fix: Start cron daemon in agent container entrypoint (#618)

The entrypoint installed a crontab but never started a cron daemon,
leaving the container idle.  Fix by running as root in the entrypoint
(cron requires it), installing the crontab for the agent user via
`crontab -u agent`, and starting cron in the foreground with `cron -f`.
Remove `USER agent` from the Dockerfile and `user: "1000:1000"` from
the compose template accordingly — cron jobs still execute as UID 1000.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/agents/Dockerfile

@@ -14,7 +14,8 @@ RUN useradd -m -u 1000 -s /bin/bash agent
 COPY entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh
 
-USER agent
+# Entrypoint runs as root to start the cron daemon;
+# cron jobs execute as the agent user (crontab -u agent).
 WORKDIR /home/agent
 
 ENTRYPOINT ["/entrypoint.sh"]