feat: implement dynamic Tailscale SSH and unified update system
Add Tailscale-based SSH connectivity and multi-system update management.
SSH Changes:
- Configure SSH to route inter-host connections through Tailscale using ProxyCommand
- Use dynamic `tailscale nc` instead of hardcoded IPs for laptop, server, macmini
- Add platform-aware Tailscale binary paths (Nix on Linux, Homebrew on macOS)
- Trust tailscale0 interface in firewalls on laptop and server
Server Infrastructure:
- Enable Tailscale service on server via new modules/server/services.nix
- Add Tailscale to macOS Homebrew casks for macmini
Update Management:
- Add `update-all` script: updates current system (flake inputs, rebuild, Homebrew, cleanup)
- Add `update-everything` script: remotely updates all online Tailscale hosts via SSH
- Add convenience aliases: update-all, update-everything
- Both scripts support NixOS and macOS with platform detection
Verification & Documentation:
- Add `verify-tailscale-ssh` script to diagnose Tailscale SSH connectivity
- Add docs/TAILSCALE-SSH.md: comprehensive Tailscale SSH setup guide
- Add docs/UPDATE-GUIDE.md: complete system update reference
This enables seamless SSH between all hosts over Tailscale's encrypted mesh network
and provides one-command updates for the entire infrastructure.