agentd: Fix docker file ownership so host tools can write to event files

Files created by containers are owned by root, blocking host-side tools (agentd kill, summary writer) from writing to them.

Fix: Run containers with --user flag to match host user: docker run --user $(id -u):$(id -g) ...

This would fix:

• t-384: agentd kill permission denied
• Summary file ownership (currently using .summary.jsonl workaround)

After this fix, we can write summaries directly to the main .jsonl file and remove the sidecar file complexity.