Add papis reference manager configuration

- Add papis.nix module with embedded configuration
- Configure library at ~/Documents/Library/papis
- Set up nvim editor and fzf picker integration
- Add comprehensive papis documentation to README

🤖 Generated with [Claude Code](https://claude.ai/code)

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

modules/papis.nix

@@ -0,0 +1,46 @@
+{ pkgs, ... }:
+
+{
+  # Install papis package
+  home.packages = with pkgs; [
+    papis
+  ];
+
+  # Papis configuration
+  home.file.".config/papis/config".text = ''
+    [settings]
+    default-library = main
+    editor = nvim
+    opentool = open
+    file-browser = open
+    
+    # Document management
+    add-folder-name = {doc[author]}-{doc[year]}-{doc[title]}
+    add-file-name = {doc[author]}-{doc[year]}-{doc[title]}
+    ref-format = {doc[author]}{doc[year]}
+    
+    # Search and display
+    sort-field = year
+    sort-reverse = True
+    match-format = {doc[tags]}{doc[author]}{doc[title]}{doc[year]}
+    
+    # Database and storage
+    database-backend = papis
+    use-git = False
+    
+    # Interface
+    fzf-binary = fzf
+    picktool = fzf
+    
+    [main]
+    dir = ~/Documents/Library/papis
+    
+    # Local configuration for the main library
+    local-config-file = .papisrc
+  '';
+
+  # Create the papis library directory
+  home.activation.createPapisDir = ''
+    mkdir -p ~/Documents/Library/papis
+  '';
+}