feat: support pnpm v12 (#288)

* feat: support pnpm v12

Approve the pinned @pnpm/exe install script for npm 12, add pnpm 12 coverage across supported runner platforms, and clarify that action-setup remains supported alongside pnpm/setup.

* refactor: use pnpm 12 native bootstrap

Install the plain pnpm v12 package as the single native bootstrap, remove the legacy @pnpm/exe lockfile and runtime path, and retain the standalone input as a no-op for workflow compatibility.

* fix: keep bootstrap updates on pnpm 12

Handle npm's array-shaped view output and resolve only pnpm 12 distribution tags.

* fix: use native bootstrap only for pnpm 12

Route pnpm 12-only versions and ranges through the plain pnpm native package while preserving the existing Node and @pnpm/exe bootstrap paths for older releases.

* style: remove updater narrative comments

package.json

@@ -14,10 +14,12 @@
     "@types/expand-tilde": "^2.0.2",
     "@types/node": "^22.0.0",
     "expand-tilde": "^2.0.2",
+    "semver": "^7.8.5",
     "yaml": "^2.3.4",
     "zod": "^3.22.4"
   },
   "devDependencies": {
+    "@types/semver": "^7.8.0",
     "esbuild": "^0.27.4",
     "typescript": "^5.3.3"
   }