serve: persist session traces across container restarts (Dockerfile VOLUME / XDG guidance)

Found during the v0.8.6 release review (Chaos/k8s persona). Deferred because it's a deployment/Dockerfile behavior change that needs design thought, not a quick fix.

Now that `altimate serve` writes session traces (#886), traces land under `$XDG_DATA_HOME/altimate-code/traces/` (default `~/.local/share/altimate-code/traces/`). In a container this is the ephemeral filesystem — a pod restart / rolling update / OOMKill silently drops the entire trace history.

Proposed:
- Document a recommended volume mount for the trace dir in the deploy/Docker docs.
- Consider `VOLUME`/`ENV XDG_DATA_HOME` in `packages/opencode/Dockerfile` (carefully — it changes default paths).
- Reference `tracing.dir` config as the override knob.