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/arch/README.md

@@ -37,10 +37,15 @@ creates and owns its directories:
 | cache | `/var/cache/tsunagi` |
 | control socket | `/run/tsunagi/agent.sock` |
 
-The polkit rule lets anyone in the `tsunagi` group configure systemd-resolved
-for the overlay zones (the service user is in that group). To run the agent as
-your own user as well, add yourself to the group: `sudo usermod -aG tsunagi
-"$USER"`.
+Installing the package enables and starts the service, and adds the user who ran
+pacman to the `tsunagi` group (log out and in again for it to apply). The
+control socket is open to that group, so members can run `tsunagi status`, the
+CLI and the tray GUI without sudo; anyone else adds themselves with `sudo
+usermod -aG tsunagi "$USER"`. The group also lets its members configure
+systemd-resolved for the overlay zones through the polkit rule.
+
+The GUI package depends on `gtk3`, `libayatana-appindicator` (the tray icon),
+`xdotool` (its `libxdo`), and the usual windowing libraries.
 
 ## Removal