Add sandboxed-workspace module for isolated dev environments

Provides isolated development environments using either VMs (microvm.nix)
or containers (systemd-nspawn) with a unified configuration interface.

Features:
- Unified options with required type field ("vm" or "container")
- Shared base configuration for networking, SSH, users, packages
- Automatic SSH host key generation and persistence
- Shell aliases for workspace management (start/stop/status/ssh)
- Automatic /etc/hosts entries for workspace hostnames
- restartIfChanged support for both VMs and containers
- Passwordless doas in workspaces

Container backend:
- Uses hostBridge for proper bridge networking with /24 subnet
- systemd-networkd for IP configuration
- systemd-resolved for DNS

VM backend:
- TAP interface with deterministic MAC addresses
- virtiofs shares for workspace directories
- vsock CID generation

common/default.nix

@@ -14,6 +14,7 @@
     ./machine-info
     ./nix-builder.nix
     ./ssh.nix
+    ./sandboxed-workspace
   ];
 
   nix.flakes.enable = true;