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/postremove.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# Remove gosh from /etc/shells on uninstall (not on upgrade).
+# deb passes "remove"/"purge", rpm passes "0" on final removal.
+set -e
+
+case "$1" in
+	remove|purge|0)
+		if command -v remove-shell >/dev/null 2>&1; then
+			remove-shell /usr/bin/gosh
+		elif [ -f /etc/shells ]; then
+			sed -i '\|^/usr/bin/gosh$|d' /etc/shells
+		fi
+		;;
+esac