Retire dotfiles-rebuild, switch to dm-pull-deploy push timer

- Drop modules/dotfiles-rebuild.nix and its imports in clan.nix;
  sunken-ship + phantom-ship no longer ship the legacy 15-min
  rebuild-from-git timer.
- Add dm-pull-deploy-push systemd timer on sunken-ship: every 15min
  runs dm-send-deploy to announce origin/main rev via data-mesher
  gossip (sunken is the dm-pull-deploy push node).
- Fix mulbo-pull service path: add openssh so 'git fetch' over an
  SSH remote stops failing with 'cannot run ssh'.
- vps-relay authorized_keys: rename Mac key comment to mac-admin,
  add sunken-ship's actual ed25519 key for ZT mesh debugging.
- home.nix: add cinny-desktop (Matrix client).
- neovim: enable cursorline.

modules/dotfiles-rebuild.nix

@@ -1,44 +0,0 @@
-# Shared auto-rebuild-from-git service for homelab hosts.
-#
-# Every 15 min: git fetch origin, fast-forward main, and if there were any
-# new commits run nixos-rebuild switch against `<dotfilesDir>#<host>`.
-#
-# Assumes /etc/dotfiles is an already-cloned checkout of the dotfiles repo.
-{ config, lib, pkgs, ... }:
-let
-  dotfilesDir = "/etc/dotfiles";
-  flakeRef = "${dotfilesDir}#${config.networking.hostName}";
-in {
-  environment.systemPackages = [ pkgs.git ];
-
-  # Trust /etc/dotfiles as root even though it's owned by `danny`.
-  # nix/libgit2 reads safe.directory from /etc/gitconfig; the GIT_CONFIG_*
-  # env vars on the service only affect the git CLI, not nix.
-  programs.git.enable = true;
-  programs.git.config.safe.directory = [ dotfilesDir ];
-
-  systemd.services.dotfiles-rebuild = {
-    description = "Pull dotfiles and run nixos-rebuild if repo changed";
-    path = with pkgs; [ git nix nixos-rebuild ];
-    environment.GIT_CONFIG_COUNT = "1";
-    environment.GIT_CONFIG_KEY_0 = "safe.directory";
-    environment.GIT_CONFIG_VALUE_0 = dotfilesDir;
-    script = ''
-      set -euo pipefail
-      cd ${dotfilesDir}
-      git fetch origin
-      if [ "$(git rev-parse HEAD)" = "$(git rev-parse origin/main)" ]; then
-        exit 0
-      fi
-      git pull origin main
-      exec nixos-rebuild switch --flake ${flakeRef}
-    '';
-    serviceConfig.Type = "oneshot";
-  };
-
-  systemd.timers.dotfiles-rebuild = {
-    wantedBy = [ "timers.target" ];
-    timerConfig.OnCalendar = "*-*-* *:00/15:00";  # every 15 minutes
-    timerConfig.RandomizedDelaySec = "2min";
-  };
-}