add goreleaser configuration for binaries and deb/rpm packages

- build static binaries for linux, darwin and freebsd
- create deb and rpm packages that register gosh in /etc/shells
- publish releases to Gitea
- add --version flag, set at build time via ldflags

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packaging/scripts/postinstall.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+# Register gosh as a login shell in /etc/shells.
+set -e
+
+if command -v add-shell >/dev/null 2>&1; then
+	add-shell /usr/bin/gosh
+elif ! grep -qx /usr/bin/gosh /etc/shells 2>/dev/null; then
+	echo /usr/bin/gosh >> /etc/shells
+fi