Arch package: start the service and let the GUI reach the agent

The control socket is now opened to the tsunagi group (unit), the Arch
package enables and starts the service on install, adds the installing user
to the group, restarts a running agent on upgrade, and depends on the
libraries the tray loads at run time. The tray looks for the agent's socket
again on every poll instead of once at start, and a missing indicator
library is reported instead of panicking. Repository URLs unified.

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

dist/linux/README.md

@@ -22,7 +22,9 @@ 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:
+`/run/tsunagi/agent.sock` with no flags. The socket is open to the `tsunagi`
+group, so after `sudo usermod -aG tsunagi "$USER"` (and logging in again) no
+`sudo` is needed:
 
 ```sh
 sudo tsunagi status