feat(zsh): replace fnm with standalone pnpm

Install the pnpm native binary from GitHub releases via gh-r and sbin,
matching the existing fnm/starship shim pattern. Node is now managed
by pnpm itself (pnpm runtime), so fnm and its init block are removed.

Drop unzip from the README optional list; only fnm's zip needed it.

README.md

@@ -5,10 +5,6 @@
 - **GNU** `stow`
 - `zsh` (I mean, duh)
 
-## Optional
-
-- `unzip` (for `fnm`)
-
 ## Installation
 
 1. clone this repo to your `$HOME` and switch to cloned dir: