fix: remove references to corepack (#10138)

Currently, our `npm publish` logic is failing.

There were a number of things that were merged recently that seemed to
contribute to this situation, though I think we have fixed most of them,
but this one stands out:

https://github.com/openai/codex/pull/10115

As best I can tell, we tried to fix the pnpm version to a specific hash,
but we did not do it consistently (though `shell-tool-mcp/package.json`
had it specified twice...), so for this PR, I ran:

```
$ git ls-files | grep package.json
codex-cli/package.json
codex-rs/responses-api-proxy/npm/package.json
package.json
sdk/typescript/package.json
shell-tool-mcp/package.json
```

and ensured that all of them now have this line:

```json
  "packageManager": "pnpm@10.28.2+sha512.41872f037ad22f7348e3b1debbaf7e867cfd448f2726d9cf74c08f19507c31d2c8e7a11525b983febc2df640b5438dee6023ebb1f84ed43cc2d654d2bc326264"
```

I also went and deleted all of the `corepack` stuff that was added by
https://github.com/openai/codex/pull/10115.

If someone can explain why we need it and verify it does not break `npm
publish`, then we can bring it back.

.github/workflows/shell-tool-mcp.yml

@@ -351,15 +351,6 @@ jobs:
         with:
           node-version: ${{ env.NODE_VERSION }}
 
-      - name: Enable Corepack
-        run: corepack enable
-
-      - name: Activate pnpm from package.json
-        run: corepack prepare --activate
-
-      - name: Verify pnpm version
-        run: pnpm --version
-
       - name: Install JavaScript dependencies
         run: pnpm install --frozen-lockfile
 
@@ -460,15 +451,6 @@ jobs:
           registry-url: https://registry.npmjs.org
           scope: "@openai"
 
-      - name: Enable Corepack
-        run: corepack enable
-
-      - name: Activate pnpm from package.json
-        run: corepack prepare --activate
-
-      - name: Verify pnpm version
-        run: pnpm --version
-
       - name: Download npm tarball
         uses: actions/download-artifact@v7
         with: