Add Finder integration to Neovim keybindings

- Add <leader>f keymap to show current file in Finder using 'open -R'
- Update README.md with new keybinding documentation
- Provides quick file location alongside existing <leader>o for opening

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

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

README.md

@@ -56,6 +56,7 @@ home-manager switch --flake github:Logan-Lin/nix-config#yanlin
 | `<Space>w` | Normal | Save file |
 | `<Space>q` | Normal | Quit |
 | `<Space>o` | Normal | Open file with system default app |
+| `<Space>f` | Normal | Show current file in Finder |
 | `<Space>y` | Normal/Visual | Copy to system clipboard |
 | `<Space>p` | Normal/Visual | Paste from system clipboard |
 | `<Space>gs` | Normal | Git status |