fix(tooling): revert corepack, pnpm is pre-installed in devcontainer image

The mcr.microsoft.com/devcontainers/typescript-node image ships pnpm
via the Node.js devcontainer feature. No corepack setup is needed.
Restore the original onCreateCommand to pnpm self-update and remove
the corepack step from copilot-instructions.md bootstrap section.

Co-authored-by: TechWatching <15186176+TechWatching@users.noreply.github.com>

Author: Copilot

.devcontainer/devcontainer.json

@@ -18,7 +18,7 @@
       "onAutoForward": "openPreview"
     }
   },
-  "onCreateCommand": "corepack enable && corepack install",
+  "onCreateCommand": "pnpm self-update",
   "updateContentCommand": "pnpm install --config.confirmModulesPurge=false",
   "postAttachCommand": "pnpm dev",
 

.github/copilot-instructions.md

@@ -22,7 +22,6 @@ The `.npmrc` sets `shamefully-hoist=true`.
 ### Bootstrap
 
 ```bash
-corepack enable        # ensure pnpm is managed by corepack
 pnpm install           # install all dependencies
 ```