replace deploy-rs CI with Comin pull-based deployment

- Add Comin service for cryodev-main (polls git repo, auto-deploys)
- Fix cryodev-pi Comin URL (cryodev-server.git -> cryodev.git)
- Remove deploy-rs from CI pipeline (was insecure with shared runner)
- Remove deploy SSH key, root SSH login, sudo rules for gitea-runner
- Revert PermitRootLogin back to 'no'
- CI now only runs flake-check + build (no deploy)
- Deployment happens via Comin (both hosts poll and self-deploy)

hosts/cryodev-pi/services/comin.nix

@@ -16,7 +16,7 @@
     remotes = [
       {
         name = "origin";
-        url = "https://${constants.services.forgejo.fqdn}/steffen/cryodev-server.git";
+        url = "https://${constants.services.forgejo.fqdn}/steffen/cryodev.git";
         branches.main.name = "main";
       }
     ];