build: manage pnpm with mise at latest

Replace the packageManager pin and .mise.toml (pnpm 12.3.4, node 26)
with mise.toml tracking pnpm latest; CI installs pnpm through mise
instead of an unpinned npm -g, keeping node from the runner image.

package.json

@@ -2,7 +2,6 @@
   "name": "aerospike",
   "version": "0.2.5",
   "description": "Simple, Opinionated Tiling Window Manager for Gnome written in Typescript",
-  "packageManager": "pnpm@12.3.4",
   "type": "module",
   "private": true,
   "repository": {