Preserve explicit config in background auto-update

[ryanfowler]

Summary

• Pass the applied explicit --config path into the background auto-update spawn path
• Keep the spawned child limited to --config, --update, --timeout=300, and --silent
• Add unit and integration coverage for config preservation during auto-update

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1