feat: --follow for deps and a custom example

--follow now follows the deptree of the definition
--follow is incompatible with --all
build_system = "custom" example with build.sh

toolchain/examples/custom-demo/build.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+# build.sh — example custom build script for zeta-makepkg.
+#
+# Invoked from the extracted source directory.  The tool sets:
+#   $DESTDIR  → staging tree root (e.g. /tmp/zeta-makepkg/mytool-.../stage)
+#
+# This script must compile/assemble the software and install every output
+# file under $DESTDIR with standard FHS paths (usr/bin/, usr/lib/, etc.).
+#
+# For a real project you would replace the example below with:
+#   - Go:      go build -o "$DESTDIR/usr/bin/mytool" .
+#   - Rust:    cargo build --release && install -D target/release/mytool "$DESTDIR/usr/bin/mytool"
+#   - Zig:     zig build -Doptimize=ReleaseSafe --prefix /usr -Ddestdir="$DESTDIR"
+#   - Shell:   install -D mytool.sh "$DESTDIR/usr/bin/mytool"
+#   - Python:  install -D mytool.py "$DESTDIR/usr/bin/mytool"
+#   - Anything you can script.
+
+set -eu
+
+echo "==> Building mytool (custom)"
+
+# Standalone Go binary: init module, build, install
+go mod init mytool
+go build -o mytool .
+
+# Install into the staging tree
+install -Dm755 mytool "$DESTDIR/usr/bin/mytool"
+
+echo "==> Install complete"