ci(deb): publish punktfunk-host .deb to the Gitea apt registry

Wires up the half-built Debian packaging: build-deb.sh existed but nothing
invoked or published it. Adds a `deb` workflow that builds the release host in
the Ubuntu 26.04 rust-ci image, packages it (dpkg-shlibdeps-resolved Depends,
NVIDIA driver filtered out), and uploads to Gitea's public Debian registry on
every main push (rolling 0.0.1~ciN.<sha>) and v* tag (clean X.Y.Z). Ubuntu hosts
then track it with `apt update && apt upgrade`.

Also: box-setup docs (packaging/debian/README.md), a pointer from the packaging
README, ignore dist/, and drop backticks from the package Description (the
unquoted control heredoc ran them as a command substitution).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/README.md

@@ -4,6 +4,9 @@ The punktfunk host is Linux-only and links system FFmpeg (NVENC), PipeWire, Opus
 the NVIDIA driver. This directory packages it for the **Fedora Atomic / Bazzite** world
 (rpm-ostree + bootc), where most of those deps are already present.
 
+> 👉 **Ubuntu/Debian hosts** install via `apt` from Gitea's package registry — see
+> [`debian/README.md`](debian/README.md) (`apt update && apt upgrade` for new builds).
+
 > 👉 **End-to-end Bazzite setup walkthrough** (install → udev/group → `host.env` → service →
 > firewall → verify → troubleshooting): [`bazzite/README.md`](bazzite/README.md). This file is the
 > higher-level packaging rationale.