#3515 swamp install.sh: holds three copies of the binary at once (~913 MB peak); delete the archive and move instead of copy

scripts/install.sh in the swamp repo (served from artifacts.swamp-club.com) downloads the release tarball into a mktemp directory (about 125 MB), extracts the binary next to it (about 394 MB, line 324), then installs a copy into the destination with install -p (line 449, another 394 MB). The temp directory is only removed by the exit trap, so the peak disk use is about 913 MB for a binary that ends up 394 MB.

That matters on disk-limited CI runners. Our Forgejo runner pods were evicted at a 1 GiB ephemeral-storage limit (swamp-club #3174), and in swamp-uat and swamp-club most jobs that install swamp (uat-cli, uat-club, tutorial, adversarial, benchmark, serve, worker, the deploys) peak at 1.3 to 2.4 GB, with this script as the largest single part.

Proposed:

• delete the archive as soon as it is extracted;
• move the extracted binary into place (mv, then chmod) when the destination is on the same filesystem, falling back to install -p otherwise; or extract straight into the destination directory.

Expected: peak about 520 MB (archive and binary briefly), steady state 394 MB. No change to what is installed or where.