Cross-compile into ./bin for the usual platforms

build.sh produced one binary for the machine it ran on. It now builds
darwin/arm64, darwin/amd64, linux/amd64 and linux/arm64 into ./bin, all
from the single version that run bumped, so the four never disagree about
what they are. PLATFORMS="linux/amd64" ./build.sh narrows it.

bin/mgsh is a symlink to the host's build, so there is still one stable
path to "the binary for this machine". The old ./mgsh in the repo root is
no longer written, and the script deletes it once if it finds it -- a
stale binary on a path people have in their fingers is worse than a
missing one.

CGO_ENABLED=0 throughout: the cross builds then need no toolchain per
target and the binaries are static. That is only safe because os/user
still resolves the current user without cgo, which mgsh needs for the
"[user@host]" stamp on every commit -- checked on darwin before relying
on it.

Windows is left out on purpose: mgsh shells out to stty and /bin/sh, so
it would compile there and then not work.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -8,4 +8,5 @@
 .TemporaryItems
 .Trashes
 mgsh
+bin/
 .mgshrc