ci: add deploy workflow and authorize CI key for yarn

- add gitea actions workflow to build and deploy on push to main
- authorize CI deploy key for root SSH on desktop
- workflow unlocks git-crypt, builds yarn config, deploys if desktop reachable

system/system-yarn.nix

@@ -64,6 +64,7 @@
 
   users.users.root.openssh.authorizedKeys.keys = [
     "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIO4jL6gYOunUlUtPvGdML0cpbKSsPNqQ1jit4E7U1RyH" # laptop
+    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIC5ZYN6idL/w/mUIfPOH1i+Q/SQXuzAMQUEuWpipx1Pc ci-deploy@muffin"
   ];
 
   programs.steam = {