Drop the run-in-background sub-agent pattern
A small CLAUDE.md-only release: the previously-recommended run_in_background=true pattern for long-running sub-agent tasks turned out to be broken by design — agents terminate after dispatching the bg task and the bg process gets killed with them. v0.3.0a19 removes that guidance and replaces it with the foreground + retry pattern that the release task uses.
No code changes, no API changes — pure documentation cleanup so future sessions don't repeat the mistake.