pdbmux: ship as k8s container, drop per-VM systemd/RPM delivery

The estate direction is all-in-kubernetes, so pdbmux (a long-running daemon)
should run as an in-cluster service rather than an RPM-installed systemd unit
on each VM. The RPM is for workstation/VM CLI tools only; a daemon does not
belong there.

- Remove packaging/pdbmux.service and drop pdbmux (binary, systemd unit,
  completions) from the RPM/nfpm spec and build-rpm.sh.
- Keep pdbmux in the Makefile build and the test suite.
- Add Dockerfile.pdbmux building a static CGO_ENABLED=0 binary on distroless
  (mirrors encapi's image style).
- Add .woodpecker/docker.yaml to build+push git.unkin.net/unkin/pdbmux:<tag>
  on v* tags via the docker-buildx plugin (droneci/DRONECI_PASSWORD creds,
  same as encapi), with k8s resources set.
- Update README/AGENTS.md: deployment is k8s, config via PDBMUX_* env.

scripts/build-rpm.sh

@@ -12,7 +12,10 @@ cd "${ROOT_DIR}"
 VERSION="${1:-${CI_COMMIT_TAG:-0.0.0-dev}}"
 VERSION="${VERSION#v}" # strip a leading v
 BINARY="node-lookup"
-BINARIES=(node-lookup pburl pblastreport pdbmux)
+# RPM ships the workstation/VM CLI tools only. pdbmux is a k8s-only daemon and
+# is deliberately excluded from the RPM (it is released as a container image);
+# it is still built + tested in this repo via the Makefile.
+BINARIES=(node-lookup pburl pblastreport)
 DIST="dist"
 
 for b in "${BINARIES[@]}"; do
@@ -37,7 +40,7 @@ export PACKAGE_VERSION="${VERSION}"
 export PACKAGE_RELEASE="1"
 export PACKAGE_ARCH="amd64"
 export PACKAGE_PLATFORM="linux"
-export PACKAGE_DESCRIPTION="CLI tools for PuppetDB: node-lookup (fact lookup/filtering), pburl and pblastreport (Puppetboard URLs and last-report times), plus pdbmux (merging proxy over the old + new PuppetDB during migration)"
+export PACKAGE_DESCRIPTION="CLI tools for PuppetDB: node-lookup (fact lookup/filtering), pburl and pblastreport (Puppetboard URLs and last-report times)"
 export PACKAGE_MAINTAINER="Ben Vincent <ben@unkin.net>"
 export PACKAGE_HOMEPAGE="https://git.unkin.net/unkin/node-lookup"
 export PACKAGE_LICENSE="MIT"