feat(pkg): add install/remove toggle with force flags

pkg now auto-detects whether each package is installed and
installs or removes accordingly. Explicit -i/--install and
-u/--uninstall flags override the auto-detection. Adds help
output when invoked without arguments.

README.md

@@ -236,7 +236,7 @@ rm -f file.txt  # Falls through to standard rm -f
 
 | Function | Description |
 |---|---|
-| `pkg <name>` | Install package: `paru -S <name>` |
+| `pkg <name>...` | Toggle package: installs if missing, removes (`-Rns`) if installed; `-i` force-install, `-u` force-uninstall |
 | `search <query>` | Search/install interactively: `paru <query>` |
 | `upgrade` | Full system upgrade: `paru -Syu --noconfirm` |
 | `cleanup` | Log and remove orphaned packages |