refactor: replace sops-nix with 1Password CLI for secrets management

modules/nixos/default.nix

@@ -5,7 +5,6 @@
     ./base.nix
     ./docker.nix
     ./locale.nix
-    ./secrets.nix
   ];
 
   # ── Default shell ──────────────────────────────────

modules/nixos/secrets.nix

@@ -1,28 +0,0 @@
-{ username, ... }:
-
-{
-  # ── sops (system-level) ─────────────────────────────
-  # Use NixOS module instead of home-manager module to avoid
-  # systemd user service issues on WSL.
-  # Secrets are placed in /run/secrets/<name>.
-  sops = {
-    age.sshKeyPaths = [ "/home/${username}/.ssh/id_ed25519" ];
-    defaultSopsFile = ../../secrets/secrets.yaml;
-    defaultSopsFormat = "yaml";
-
-    secrets = {
-      ai_gateway_base_url = {
-        owner = username;
-      };
-      ai_gateway_api_key = {
-        owner = username;
-      };
-      exa_api_key = {
-        owner = username;
-      };
-      context7_api_key = {
-        owner = username;
-      };
-    };
-  };
-}