fix: respect directory parameter in machines_dir

The machines_dir() function was hardcoding "machines" without considering
the directory parameter from buildClan/clan configuration. This caused
update-hardware-config and other commands to write files to the wrong
location when a custom directory was specified (e.g., directory = ./clan).

Solution:
1. Added relativeDirectory to inventoryClass in Nix, computed where both
   self and directory have consistent store paths during evaluation
2. Updated machines_dir() to use this pre-computed relative path from Nix
   via flake.select("clanInternals.inventoryClass.relativeDirectory")

Fixes: https://git.clan.lol/clan/clan-core/issues/2906

modules/inventoryClass/default.nix

@@ -81,6 +81,14 @@ in
     directory = mkOption {
       type = types.path;
     };
+    relativeDirectory = mkOption {
+      type = types.str;
+      internal = true;
+      description = ''
+        The relative directory path from the flake root to the clan directory.
+        Empty string if directory equals the flake root.
+      '';
+    };
     machines = mkOption {
       type = types.attrsOf (submodule ({
         options = {