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/sandboxed-workspace/base.nix

@@ -0,0 +1,114 @@
+{ hostConfig, workspaceName, ip, networkInterface }:
+
+# Base configuration shared by all sandboxed workspaces (VMs and containers)
+# This provides common settings for networking, SSH, users, and packages
+#
+# Parameters:
+#   hostConfig      - The host's NixOS config (for inputs, ssh keys, etc.)
+#   workspaceName   - Name of the workspace (used as hostname)
+#   ip              - Static IP address for the workspace
+#   networkInterface - Match config for systemd-networkd (e.g., { Type = "ether"; } or { Name = "host0"; })
+
+{ config, lib, pkgs, ... }:
+
+{
+  imports = [
+    ../shell.nix
+    hostConfig.inputs.home-manager.nixosModules.home-manager
+    hostConfig.inputs.nix-index-database.nixosModules.default
+  ];
+
+  # Basic system configuration
+  system.stateVersion = "25.11";
+
+  # Set hostname to match the workspace name
+  networking.hostName = workspaceName;
+
+  # Networking with systemd-networkd
+  networking.useNetworkd = true;
+  systemd.network.enable = true;
+
+  # Enable resolved to populate /etc/resolv.conf from networkd's DNS settings
+  services.resolved.enable = true;
+
+  # Basic networking configuration
+  networking.useDHCP = false;
+
+  # Static IP configuration
+  # Uses the host as DNS server (host forwards to upstream DNS)
+  systemd.network.networks."20-workspace" = {
+    matchConfig = networkInterface;
+    networkConfig = {
+      Address = "${ip}/24";
+      Gateway = hostConfig.networking.sandbox.hostAddress;
+      DNS = [ hostConfig.networking.sandbox.hostAddress ];
+    };
+  };
+
+  # Disable firewall inside workspaces (we're behind NAT)
+  networking.firewall.enable = false;
+
+  # Enable SSH for access
+  services.openssh = {
+    enable = true;
+    settings = {
+      PasswordAuthentication = false;
+      PermitRootLogin = "prohibit-password";
+    };
+  };
+
+  # Use persistent SSH host keys from shared directory
+  services.openssh.hostKeys = lib.mkForce [
+    {
+      path = "/etc/ssh-host-keys/ssh_host_ed25519_key";
+      type = "ed25519";
+    }
+  ];
+
+  # Basic system packages
+  environment.systemPackages = with pkgs; [
+    kakoune
+    vim
+    git
+    htop
+    wget
+    curl
+    tmux
+    dnsutils
+  ];
+
+  # User configuration
+  users.mutableUsers = false;
+  users.users.googlebot = {
+    isNormalUser = true;
+    extraGroups = [ "wheel" ];
+    shell = pkgs.fish;
+    openssh.authorizedKeys.keys = hostConfig.machines.ssh.userKeys;
+  };
+
+  security.doas.enable = true;
+  security.sudo.enable = false;
+  security.doas.extraRules = [
+    { groups = [ "wheel" ]; noPass = true; }
+  ];
+
+  # Minimal locale settings
+  i18n.defaultLocale = "en_US.UTF-8";
+  time.timeZone = "America/Los_Angeles";
+
+  # Enable flakes
+  nix.settings.experimental-features = [ "nix-command" "flakes" ];
+
+  # Make nixpkgs available in NIX_PATH and registry (like the NixOS ISO)
+  # This allows `nix-shell -p`, `nix repl '<nixpkgs>'`, etc. to work
+  nix.nixPath = [ "nixpkgs=${hostConfig.inputs.nixpkgs}" ];
+  nix.registry.nixpkgs.flake = hostConfig.inputs.nixpkgs;
+
+  # Enable fish shell
+  programs.fish.enable = true;
+
+  # Home Manager configuration
+  home-manager.useGlobalPkgs = true;
+  home-manager.useUserPackages = true;
+  home-manager.users.googlebot = import ./home.nix;
+}