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>

bin/disinto

@@ -192,7 +192,6 @@ services:
   agents:
     build: ./docker/agents
     restart: unless-stopped
-    user: "1000:1000"
     volumes:
       - agent-data:/home/agent/data
       - project-repos:/home/agent/repos