feat: replace local opencode with upstream flake input v1.1.27

- Add `opencode` flake input pinned to v1.1.27 for stability
- Remove local `pkgs/opencode` derivation and overlay
- Add `opencode` and `opencode-desktop` to `pkgs/default.nix` from input

overlays/mods/default.nix

@@ -3,7 +3,6 @@
   # This overlay contains package overrides and modifications
 
   # n8n = import ./n8n.nix {inherit prev;};
-  # opencode = import ./opencode.nix {inherit prev;};
   # beads = import ./beads.nix {inherit prev;};
 
   # Add more modifications here as needed

overlays/mods/opencode.nix

@@ -1,16 +0,0 @@
-{prev}:
-prev.opencode.overrideAttrs (oldAttrs: rec {
-  version = "1.1.18";
-
-  src = prev.fetchFromGitHub {
-    owner = "anomalyco";
-    repo = "opencode";
-    tag = "v${version}";
-    hash = "sha256-3A4s0FpjZuGB0HGMQVBXfWq+0yHmeIvnEQTSX3amV4I=";
-  };
-
-  node_modules = oldAttrs.node_modules.overrideAttrs (old: {
-    inherit version src;
-    outputHash = "sha256-zSco4ORQQOqV3vMPuP+M/q/hBa+MJGnTKIlxgngMA3g=";
-  });
-})