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/hosts/nixos/nixnuc/default.nix

@@ -11,8 +11,8 @@ in {
     ./containers/mountain-mesh-bot-discord.nix
     ./containers/psitransfer.nix
     ./monitoring-stack.nix
-    ../../common/linux/lets-encrypt.nix
-    ../../common/linux/restic.nix
+    ../../../shared/nixos/lets-encrypt.nix
+    ../../../shared/nixos/restic.nix
   ];
 
   system.stateVersion = "23.11";

modules/hosts/nixos/nixnuc/monitoring-stack.nix

@@ -363,7 +363,7 @@ in {
       uptimekuma_grafana_api_key = {
         owner = "vmagent";
         restartUnits = ["vmagent.service"];
-        sopsFile = ../../common/secrets.yaml;
+        sopsFile = ../../../shared/secrets.yaml;
       };
     };
   };