Add update flake and skill creator skills

.claude/skills/update-flake/references/machines.md

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+# Machine Build Reference
+
+## Listing Machines
+
+Get the current list dynamically:
+```bash
+nix eval .#nixosConfigurations --apply 'x: builtins.attrNames x' --json
+```
+
+## Building a Machine
+
+```bash
+nix build .#nixosConfigurations.<hostname>.config.system.build.toplevel --no-link
+```
+
+## Important Constraints
+
+- **stateVersion**: If any update requires incrementing `system.stateVersion` or `home.stateVersion`, or any data migration, STOP and ask the user. Do not proceed on your own.
+- **nextcloud**: Pinned to a specific version (e.g. `pkgs.nextcloud32`). Only upgrade one major version at a time. Notify the user when upgrading.