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