`rea doctor --target ""` passed an empty string to target readiness,
which checked the working directory instead of reporting that no app
was named. Use the shared local path schema, so an empty target fails
validation before any readiness check, as every other target does.
Fixes#7561
Co-Authored-By: gensokyo-dev <gensokyo-dev@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
