Add Arch Linux PKGBUILD and package build step

- PKGBUILD: builds converge package, uses pre-built binary from
  build/converge when available (from scripts/build), falls back to
  stack build from source. Installs binary, systemd user service,
  example config, and README.
- scripts/build: added final step to run makepkg -f on the host
  (conditional, skipped if makepkg not found). Produces .pkg.tar.zst
  alongside the binary.
- scripts/makepkg: convenience wrapper for standalone makepkg runs.

scripts/makepkg

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Build an Arch Linux package for converge using makepkg.
+#
+# Usage:
+#   ./scripts/makepkg            # Build the package (no install)
+#   ./scripts/makepkg -i         # Build and install
+#   ./scripts/makepkg -s         # Build and install missing deps
+#
+# The resulting .pkg.tar.zst will be placed in the project root.
+set -euo pipefail
+cd "$(dirname "${BASH_SOURCE[0]}")/.."
+
+makepkg -f "$@"