Inconsistent rules for Command output

The `StdoutFile` and `StderrFile` fields of `Command` are used as-is when opening the files they name. But when specified via yaml, the filenames are interpreted relative to the yaml file's `_dir` directive.

Start by tracing how Command's StdoutFile and StderrFile values are opened, comparing direct field values with values loaded from YAML. Check how the YAML _dir directive affects filenames. Done means both fields follow one consistent path-resolution rule, with the behavior verified for direct and YAML-specified commands.

Written by the indexing model from the issue text.