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.lock

@@ -0,0 +1,82 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1771207753,
+        "narHash": "sha256-b9uG8yN50DRQ6A7JdZBfzq718ryYrlmGgqkRm9OOwCE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "d1c15b7d5806069da59e819999d70e1cec0760bf",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1771470520,
+        "narHash": "sha256-PvytHcaYN5cPUll7FB70mXv1rRsIBRmu47fFfq3haxA=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "a1d4cc1f264c45d3745af0d2ca5e59d460e58777",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}