Add release machinery: version bump, nfpm RPM, release-on-tag pipeline

- Makefile: add make patch|minor|major (tag + push), dist-build, completions,
  and rpm/rpm-package targets.
- packaging/nfpm.yaml + scripts/build-rpm.sh: package the tomswall binary with
  bash/zsh completions, the example config, and a systemd agent unit into an RPM.
- packaging/tomswall-agent.service + agent.env: run `tomswall agent` as a
  systemd service (CAP_NET_ADMIN/CAP_NET_RAW), configured via /etc/tomswall/agent.env.
- .woodpecker/release.yaml: on v* tag, test -> build -> package RPM -> PUT to the
  artifactapi rpm-internal repo. Matches node-lookup conventions.

scripts/build-rpm.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+#
+# Package the (already built) tomswall binary into an RPM with nfpm, bundling
+# generated bash/zsh shell completions and the systemd agent unit.
+# Usage: scripts/build-rpm.sh [version]   (version defaults to $CI_COMMIT_TAG)
+#
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "${ROOT_DIR}"
+
+VERSION="${1:-${CI_COMMIT_TAG:-0.0.0-dev}}"
+VERSION="${VERSION#v}" # strip a leading v
+BINARY="tomswall"
+DIST="dist"
+
+if [ ! -f "${DIST}/${BINARY}" ]; then
+  echo "ERROR: ${DIST}/${BINARY} not found; run 'make dist-build' first" >&2
+  exit 1
+fi
+
+# Generate shell completions from the freshly built binary so they always match
+# the shipped flags/subcommands.
+COMP_DIR="${DIST}/completions"
+mkdir -p "${COMP_DIR}"
+"./${DIST}/${BINARY}" completion bash >"${COMP_DIR}/${BINARY}.bash"
+"./${DIST}/${BINARY}" completion zsh >"${COMP_DIR}/_${BINARY}"
+
+export PACKAGE_NAME="${BINARY}"
+export PACKAGE_VERSION="${VERSION}"
+export PACKAGE_RELEASE="1"
+export PACKAGE_ARCH="amd64"
+export PACKAGE_PLATFORM="linux"
+export PACKAGE_DESCRIPTION="Spiritual successor to shorewall — nftables firewall manager, with a control-plane agent that pulls compiled config from tomswallapi"
+export PACKAGE_MAINTAINER="Ben Vincent <ben@unkin.net>"
+export PACKAGE_HOMEPAGE="https://git.unkin.net/unkin/tomswall"
+export PACKAGE_LICENSE="MIT"
+
+envsubst <packaging/nfpm.yaml >"${DIST}/nfpm.yaml"
+nfpm pkg --config "${DIST}/nfpm.yaml" --target "${DIST}" --packager rpm
+
+echo "Built:"
+ls -1 "${DIST}"/*.rpm