Add paper CLI: download academic papers by DOI and convert to markdown

Downloads PDFs from LibGen (primary) or Anna's Archive API (fallback),
converts to markdown via marker_single, and prints to stdout. Includes
XDG-compliant caching, nix flake with marker-pdf packaging, and a
Claude Code skill for paper-reader integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -0,0 +1,55 @@
+{
+  description = "paper — download papers by DOI and convert to markdown";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, flake-utils, rust-overlay }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs { inherit system overlays; };
+        rust-nightly = pkgs.rust-bin.nightly.latest.default.override {
+          extensions = [ "rust-src" "rust-analyzer" ];
+        };
+
+        marker = import ./nix/marker.nix { inherit pkgs; };
+
+        paper = pkgs.rustPlatform.buildRustPackage {
+          pname = "paper";
+          version = "0.1.0";
+          src = pkgs.lib.cleanSource ./.;
+          cargoLock.lockFile = ./Cargo.lock;
+        };
+
+        # Wrap the paper binary so marker_single is on PATH
+        paper-wrapped = pkgs.symlinkJoin {
+          name = "paper-${paper.version}";
+          paths = [ paper ];
+          nativeBuildInputs = [ pkgs.makeWrapper ];
+          postBuild = ''
+            wrapProgram $out/bin/paper \
+              --prefix PATH : ${pkgs.lib.makeBinPath [ marker.markerEnv ]}
+          '';
+        };
+      in
+      {
+        packages = {
+          default = paper-wrapped;
+          unwrapped = paper;
+        };
+
+        devShells.default = pkgs.mkShell {
+          buildInputs = [
+            rust-nightly
+            marker.markerEnv
+          ];
+        };
+      });
+}