chore: let Corepack provision pnpm from package.json as single source of truth

Remove the duplicated PNPM_VERSION pin from devcontainer.json build args and
the Dockerfile ARG (which had drifted to 11.5.3 vs package.json's 11.5.0).
Corepack now reads package.json#packageManager at runtime; set
COREPACK_ENABLE_DOWNLOAD_PROMPT=0 so first-use provisioning is non-interactive.
Update docs accordingly.

.devcontainer/devcontainer.json

@@ -5,10 +5,7 @@
   // Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
   "build": {
     "dockerfile": "Dockerfile",
-    "context": ".",
-    "args": {
-      "PNPM_VERSION": "11.5.3"
-    }
+    "context": "."
   },
 
   // Features to add to the dev container. More info: https://containers.dev/features.