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>

build.sh

@@ -1,12 +1,22 @@
 #!/bin/sh
-# Build mgsh, auto-incrementing the patch version by 0.0.1 on every build.
+# Build mgsh for the usual platforms into ./bin, auto-incrementing the patch
+# version by 0.0.1 on every build.
 #
 # version.txt holds the currently built version. Each run increments the patch
-# component, then builds with that version injected via -ldflags, and writes it
-# back. So version.txt always reflects the version of the binary just built.
+# component, then builds every platform with that one version injected via
+# -ldflags, and writes it back. So version.txt always reflects the version of
+# the binaries just built, and all of them carry the same one.
+#
+# Override the platform list to build just one:
+#   PLATFORMS="linux/amd64" ./build.sh
+#
+# Windows is deliberately absent: mgsh shells out to stty and /bin/sh, so it
+# would compile there and then not work.
 set -e
 cd "$(dirname "$0")"
 
+PLATFORMS=${PLATFORMS:-"darwin/arm64 darwin/amd64 linux/amd64 linux/arm64"}
+
 V=$(cat version.txt 2>/dev/null || echo 4.0.0)
 
 # split MAJOR.MINOR.PATCH and increment PATCH (no carry: 4.0.9 -> 4.0.10)
@@ -17,7 +27,34 @@ PATCH=${REST#*.}
 PATCH=$((PATCH + 1))
 NV="$MAJOR.$MINOR.$PATCH"
 
-go build -ldflags "-X main.VERSION=$NV" -o mgsh .
+# earlier versions built ./mgsh in the repo root; drop it so nothing keeps
+# running a stale binary from a path that is no longer written
+if [ -f mgsh ]; then
+	rm -f mgsh
+	echo "removed stale ./mgsh (the build now writes ./bin)"
+fi
+
+mkdir -p bin
+HOST="$(go env GOOS)/$(go env GOARCH)"
+
+for p in $PLATFORMS; do
+	os=${p%/*}
+	arch=${p#*/}
+	out="bin/mgsh-$os-$arch"
+
+	# CGO_ENABLED=0 throughout: it makes the cross builds work without a
+	# toolchain per target and the binaries static, and os/user still resolves
+	# the current user without cgo on both darwin and linux.
+	CGO_ENABLED=0 GOOS="$os" GOARCH="$arch" \
+		go build -ldflags "-X main.VERSION=$NV" -o "$out" .
+
+	if [ "$p" = "$HOST" ]; then
+		ln -sf "mgsh-$os-$arch" bin/mgsh # the one for this machine
+		echo "  $out  ->  bin/mgsh"
+	else
+		echo "  $out"
+	fi
+done
 
 echo "$NV" > version.txt
 echo "built mgsh v$NV"