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.

nixos/neovim.nix

@@ -41,6 +41,7 @@
       end
 
       -- General options
+      vim.opt.cursorline = true
       vim.opt.mouse = "a"
       vim.opt.listchars = { tab = "→ ", space = "·", nbsp = "␣", trail = "•", eol = "¶", precedes = "«", extends = "»" }
       vim.opt.clipboard:append("unnamedplus")