Add packaging, a CI workflow, and containerised local runs

Add packaging, a CI workflow, and containerised local runs

A v* tag now builds a .deb and a .rpm for amd64 and arm64 with nfpm and attaches them, with checksums, to the release. The package ships the systemd unit, a config at /etc/vole/vole.toml, and a postinstall that creates the service account and prepares the instance that config names; `vole admin init --config` is what makes that possible, and the binary's version is stamped from the tag instead of the source tree.

scripts/ci.sh becomes .vole/workflows/ci.yaml, so `vole workflow run ci` executes exactly what a push runs, and the Markdown link check moves to scripts/check-links.py. Local runs now always execute in the job image and share ci/container with the runner; --no-container is gone, because a run in a different environment proves nothing about CI.

The runner mounts a per-repository namespace at /cache and sets no variable for it: a job opts in through its image's env or the new job-level env, which is pinned with the queue and delivered with the lease. Whole namespaces are evicted least-recently-written first so a module cache is never half-deleted, and contributor branches get their own namespace so an untrusted tip cannot seed the cache a protected branch builds into.

Running the suite in that container found four bugs worth naming: /tmp was mounted noexec, which broke `go test` and `go run`; the read-only root is gone, because images make toolchain directories world-writable deliberately and the mount set is what protects the host; OpenSSH needs a passwd entry for the job's uid; and a lost metadata push reported as "cannot lock ref" was not classified as a race, so concurrent CLI writes gave up on the first collision. That retry now backs off exponentially with jitter.