Summary

• run the shell smoke test under a temporary isolated HOME instead of the developer's real ~/.nullhub state
• fail immediately when the background nullhub process exits during startup or any request, instead of reporting a false-positive pass
• capture the background server log and print it on unexpected exit to make smoke failures actionable
• clean up the temporary test home on exit so the smoke run leaves no local state behind

Validation

• npm --prefix ui ci --no-audit --no-fund
• npm --prefix ui run build
• zig build test -Dbuild-ui=false --summary all
• bash tests/test_e2e.sh
• zig fmt --check src/

Notes

• this PR is intentionally limited to the smoke harness and does not touch the underlying boot reconciliation logic
• isolating HOME also avoids accidental coupling to developer-local managed instances, cached binaries, or stale state files
• the clean-worktree validation exposed a separate build quirk: build.zig can panic if ui/build is missing and it tries to spawn npm itself; this PR does not change that behavior