Add system default app integration to Neovim

- Add <leader>o keymap to open current file with system default application
- Uses ':silent \!open %' to prevent command output clutter
- Update README with new keymap documentation
- Enables quick preview of markdown, images, PDFs in native apps

Enhances workflow by bridging Neovim editing with system-level file viewing.

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

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

nvim.nix

@@ -132,6 +132,14 @@
         options = { desc = "Replace selection with system clipboard"; };
       }
 
+      # System integration
+      {
+        mode = "n";
+        key = "<leader>o";
+        action = ":silent !open %<CR>";
+        options = { desc = "Open file with system default app"; };
+      }
+
       # Git keymaps (vim-fugitive)
       {
         mode = "n";