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

src/install-pnpm/bootstrap/pnpm-lock.json

@@ -5,13 +5,13 @@
   "packages": {
     "": {
       "dependencies": {
-        "pnpm": "11.19.0"
+        "pnpm": "11.25.0"
       }
     },
     "node_modules/pnpm": {
-      "version": "11.19.0",
-      "resolved": "https://registry.npmjs.org/pnpm/-/pnpm-11.19.0.tgz",
-      "integrity": "sha512-eIHz7VkNRyxKlV4riLISF5ERYGbcyIy8o4SeybYPG7qm0syyIfqR2k4cZb7yvL43k2Wup6xTnHv4be3DobItzg==",
+      "version": "11.25.0",
+      "resolved": "https://registry.npmjs.org/pnpm/-/pnpm-11.25.0.tgz",
+      "integrity": "sha512-XN6SW08HX3Jetx+64YpC/+eEUkeJ8ZthxzHLhyHsKKruFg4BqNWvT+2ypCzb8wDv4j2zVrDUoXtNY+EfirfJVg==",
       "license": "MIT",
       "bin": {
         "pn": "bin/pnpm.mjs",