[puncsky]

Summary

Noninteractive psql starts its child without stdin and discards stderr when the command succeeds. Forward Cobra's input reader to the child and send successful WARNING/NOTICE output to Cobra's stderr writer while keeping stdout as the structured result.

Motivation

Piped COPY / pstdin imports can read EOF and import no rows, and successful SQL can silently lose important diagnostics. The same subprocess helper now preserves both streams; nonzero child errors retain their existing error path.

How to verify

go test ./cmd/... ./pkg/... ./internal/...

• All 57 development packages pass on the candidate.
• Real child-process tests cover exact CSV/Unicode/newline input, empty input, commands that do not read input, stdout plus warning, notice-only success, and nonzero errors. Removing stdin forwarding or diagnostics forwarding fails the corresponding behavioral tests.
• New-code lint is clean against c31e586e; full lint on unmodified main reports 92 pre-existing findings with the pinned golangci-lint v2.14.0.

Release notes

Noninteractive psql forwards piped input and preserves successful PostgreSQL warnings and notices on stderr.