replace: last in pipeline silently drops every record #106

Bug

If replace has no output channel (it is last in the pipeline), Run skips the record loop entirely and returns nil. Records are dropped with no error. sample rejects being last via ErrInvalidConfig; replace does not.

Where

internal/pkg/pipeline/task/replace/replace.go — the processing loop is inside if output != nil.

Expected

Return an error when output == nil, or still apply the replacement and write somewhere observable. Silent success with no output is the worst of both.

Found while auditing task READMEs in #96.