Makes explorer and desktop sibling projects, as proposed in ADR 1.
This commit only moves files; paths in CI, scripts and docs are fixed
in the following commits.
The non-local path leaves EXTRA_ENV as an empty array, and expanding
"${EXTRA_ENV[@]}" under `set -u` errors with "unbound variable" on
bash 3.2 (the version shipped on macOS), aborting before the suite
runs. Guard the expansion with ${EXTRA_ENV[@]+...} so an empty array
expands to nothing instead.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Rūdolfs Ošiņš