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/home/danny/home.nix

@@ -231,6 +231,7 @@
     zed-editor
     code-cursor
     cursor-cli
+    cinny-desktop # Matrix client (Tauri wrapper around the Cinny web app)
     dfu-util      # USB DFU firmware flasher (Flipper Zero etc.)
     discord
     mapscii