Package as a dedicated tsunagi user; add Arch packages

Reworks the Linux packaging so the service runs as an unprivileged, dedicated
"tsunagi" user instead of root — the point of a native package is to set things
up properly for that OS's users — and adds Arch Linux packages beside the deb.
Both package families are built from the already-compiled static binary; nothing
is recompiled.

Service model (shared systemd unit, dist/linux/tsunagi.service):
- User=tsunagi with AmbientCapabilities=CAP_NET_ADMIN CAP_NET_BIND_SERVICE, so
  it creates the TUN and binds low ports without root.
- StateDirectory/CacheDirectory/RuntimeDirectory give it /var/lib/tsunagi,
  /var/cache/tsunagi and /run/tsunagi, and the agent is pointed at them (and the
  socket at /run/tsunagi/agent.sock) through TSUNAGI_* env in the unit.
- The polkit rule now grants the resolve1 actions to the "tsunagi" group; the
  service user is in it, and admins can add themselves.

CLI: when no state dir is chosen, the control commands prefer the system
service's socket at /run/tsunagi/agent.sock when it exists, so `sudo tsunagi
status`/`join` reach the packaged daemon with no flags. A per-user agent (its
socket absent from /run) still uses its own.

Packaging:
- deb (build-deb.sh): postinst now creates the tsunagi system user+group; unit
  installed to /lib/systemd/system, binary to /usr/bin.
- Arch (new dist/arch/build-pkg.sh): a dependency-free .pkg.tar.zst assembled
  with bsdtar + zstd (no makepkg, no Arch host) — .PKGINFO, a .INSTALL scriptlet
  that creates the user and reloads systemd, and a gzipped .MTREE; unit to
  /usr/lib/systemd/system.
- The polkit rule moved to dist/linux (shared by deb, Arch and the tarball), and
  the tarball install.sh now creates the user and installs the unit + rule.
- Both release workflows build the deb and the Arch package on the two Linux
  matrix entries (deb_arch/pkg_arch), install libarchive-tools+zstd for it, and
  attach both to the release (Gitea API loop / gh).

Not exercised in this session: the .deb and .pkg.tar.zst build and install only
on Linux (no dpkg-deb/bsdtar+pacman locally); CLI build, clippy, fmt and tests
pass, and all shell/YAML is syntax-checked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dist/linux/README.md

@@ -2,40 +2,45 @@
 
 This archive holds a **static** `tsunagi` binary (musl), so it runs on any
 distribution — Arch, Alpine, NixOS, old glibc systems — with no shared-library
-dependencies. Alongside it are a systemd unit and an installer.
+dependencies. Alongside it are a systemd unit, a polkit rule, and an installer.
 
 Creating the TUN interface needs `CAP_NET_ADMIN`, and configuring
-systemd-resolved is decided by polkit (by user id, not capability). There are
-three ways to satisfy that; pick one.
+systemd-resolved is decided by polkit. There are three ways to run it; pick one.
 
-## Quick install — systemd service as root
+> On Debian/Ubuntu and Arch, prefer the native package (`.deb` / `.pkg.tar.zst`)
+> from the release — it does all of the below for you.
+
+## Quick install — systemd service as a dedicated user
 
 ```sh
 sudo ./install.sh
 sudo systemctl enable --now tsunagi
 ```
 
-The service runs the agent as root, so the interface, low ports and the
-resolver all just work (root is not subject to polkit). Control it **as root**,
-so the client resolves the same state directory the daemon uses:
+The service runs as an unprivileged `tsunagi` user with just `CAP_NET_ADMIN`
+and `CAP_NET_BIND_SERVICE` (granted by systemd), not root. systemd creates its
+state (`/var/lib/tsunagi`), cache (`/var/cache/tsunagi`) and runtime
+(`/run/tsunagi`) directories, and the polkit rule lets it configure the
+resolver. Control it with `sudo` — the CLI finds the running service's socket at
+`/run/tsunagi/agent.sock` with no flags:
 
 ```sh
 sudo tsunagi status
 sudo tsunagi join <network-name> <tsn1…secret>
 ```
 
-## As your own user — one capability, no root at runtime
+## As your own user — one capability, no service
 
 ```sh
 sudo install -m 0755 tsunagi /usr/local/bin/tsunagi
 sudo setcap cap_net_admin,cap_net_bind_service+p /usr/local/bin/tsunagi
+sudo usermod -aG tsunagi "$USER"   # so the resolver works; log in again after
 tsunagi up
 ```
 
-Nothing is left behind and nothing runs as root. The capability is lost on every
-rebuild or copy of the binary, so re-run `setcap` after replacing it. For the
-local DNS resolver, run `tsunagi dns` once: it prints the exact polkit rule that
-lets your user configure systemd-resolved, ready to paste.
+Nothing runs as root. The capability is lost on every rebuild or copy of the
+binary, so re-run `setcap` after replacing it. The agent uses your per-user
+state directory and socket, so plain `tsunagi status` (no sudo) reaches it.
 
 ## Without touching the OS