Restructure modules/shared and update all imports

- Delete 2024-12-rework/ folder
- Rename modules/hosts/common to modules/shared, then split into:
  - modules/shared/home/general
  - modules/shared/home/linux
  - modules/shared/nixos
- Update all import paths throughout the codebase
- Update lib/*.nix to use new paths
- Fix hardcoded /Users/${username} to use config.home.homeDirectory
- Update README and examples/flake-structure.nix

modules/shared/nixos/lets-encrypt.nix

@@ -0,0 +1,25 @@
+{ config, username, ... }: {
+
+  ##########################################################################
+  #                                                                        #
+  #  This module sets up Let's Encrypt certs via a DNS challenge to Gandi  #
+  #                                                                        #
+  ##########################################################################
+
+  security.acme = {
+    acceptTerms = true;
+    defaults = {
+      email = "lets-encrypt@technicalissues.us";
+      credentialFiles = { "GANDIV5_PERSONAL_ACCESS_TOKEN_FILE" = "${config.sops.secrets.gandi_dns_pat.path}"; };
+      dnsProvider = "gandiv5";
+      dnsResolver = "ns1.gandi.net";
+      # uncomment below for testing
+      #server = "https://acme-staging-v02.api.letsencrypt.org/directory";
+    };
+  };
+
+  sops = {
+    age.keyFile = "${config.users.users.${username}.home}/.config/sops/age/keys.txt";
+    secrets.gandi_dns_pat.sopsFile = ../secrets.yaml;
+  };
+}