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

machines/fry/workspaces/test-container.nix

@@ -0,0 +1,20 @@
+{ config, lib, pkgs, ... }:
+
+# Test container workspace configuration
+#
+# Add to sandboxed-workspace.workspaces in machines/fry/default.nix:
+#   sandboxed-workspace.workspaces.test-container = {
+#     type = "container";
+#     config = ./workspaces/test-container.nix;
+#     ip = "192.168.83.50";
+#   };
+#
+# The workspace name ("test-container") becomes the hostname automatically.
+# The IP is configured in default.nix, not here.
+
+{
+  # Install packages as needed
+  environment.systemPackages = with pkgs; [
+    # Add packages here
+  ];
+}