Add Nix flake for Python 2.7 development environment

Uses nixpkgs-18.09 which has all required Python 2 packages:
- fuse-python 0.2.1
- mutagen 1.41.1
- beets 1.4.9 (built from source)
- jellyfish 0.6.1
- munkres 1.0.6

Run 'nix develop' or 'direnv allow' to enter the environment.

.envrc

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+use flake

flake.lock

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+{
+  "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-old": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1558254092,
+        "narHash": "sha256-5v6XuO9dOVpB3ZGNyDvLqOvCnzlyyvscTYbNiQouSZo=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "a7e559a5504572008567383c3dc8e142fa7a8633",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-18.09",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs-old": "nixpkgs-old"
+      }
+    },
+    "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
+}

flake.nix

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+{
+  description = "beetfs - FUSE filesystem for beets with metadata overlay (Python 2.7)";
+
+  inputs = {
+    # Using nixos-18.09 - has all Python 2 packages: fuse, mutagen, jellyfish, munkres
+    # Mark as non-flake since 18.09 predates flakes
+    nixpkgs-old = {
+      url = "github:NixOS/nixpkgs/nixos-18.09";
+      flake = false;
+    };
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs-old, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs-old {
+          inherit system;
+        };
+
+        # Build beets 1.4.9 for Python 2 (last Python 2 compatible version)
+        # Using minimal deps - autotagger features (munkres, jellyfish) not needed for beetfs
+        beets-py2 = pkgs.python2Packages.buildPythonPackage rec {
+          pname = "beets";
+          version = "1.4.9";
+
+          # Use legacy setup.py install (not pip/wheel)
+          format = "other";
+
+          src = pkgs.fetchFromGitHub {
+            owner = "beetbox";
+            repo = "beets";
+            rev = "v${version}";
+            sha256 = "sha256-KO5jtqxw82ylbSKsJgUGr3bfxUPH8vanf/+kv//CreM=";
+          };
+
+          # No patching needed - nixpkgs-18.09 has all required Python 2 packages
+
+          nativeBuildInputs = with pkgs.python2Packages; [ setuptools ];
+
+          buildPhase = "true";  # No build needed
+
+          installPhase = ''
+            # Direct copy to avoid dependency resolution
+            mkdir -p $out/lib/python2.7/site-packages
+            cp -r beets $out/lib/python2.7/site-packages/
+            cp -r beetsplug $out/lib/python2.7/site-packages/
+
+            # Create version file
+            echo "__version__ = '${version}'" >> $out/lib/python2.7/site-packages/beets/__init__.py
+
+            # Create beet command wrapper
+            mkdir -p $out/bin
+            cat > $out/bin/beet << 'EOF'
+#!${pkgs.python2.interpreter}
+import sys
+from beets.ui import main
+main()
+EOF
+            chmod +x $out/bin/beet
+          '';
+
+          propagatedBuildInputs = with pkgs.python2Packages; [
+            pyyaml
+            mutagen
+            unidecode
+            enum34
+            six
+            musicbrainzngs
+            jellyfish
+            munkres
+          ];
+
+          # Disable tests for faster builds
+          doCheck = false;
+
+          meta = {
+            description = "Music library manager and tagger";
+            homepage = "https://beets.io/";
+          };
+        };
+
+        pythonEnv = pkgs.python2.withPackages (ps: with ps; [
+          fuse
+          mutagen
+          pyyaml
+          enum34
+          six
+          jellyfish
+          munkres
+          unidecode
+          musicbrainzngs
+        ] ++ [ beets-py2 ]);
+
+      in {
+        devShells.default = pkgs.mkShell {
+          buildInputs = [
+            pythonEnv
+            pkgs.fuse
+          ];
+
+          shellHook = ''
+            # Clear any system Python pollution and set clean PYTHONPATH
+            unset PYTHONPATH
+            export PYTHONPATH="$PWD/beetsplug"
+            echo "beetfs development environment (Python 2.7)"
+            echo "  Python: $(python --version 2>&1)"
+            echo "  Run: python -c 'import beets; print(beets.__version__)'"
+            echo ""
+            echo "To mount: beet mount <mountpoint>"
+          '';
+        };
+      }
+    );
+}