Fix pnpm version conflict between package.json and CI workflow

[Copilot]

The CI workflow specified version: 9 in pnpm/action-setup@v4 while package.json declared "packageManager": "pnpm@9.15.9", causing a version conflict error.

Changes

• Removed explicit version: 9 from .github/workflows/ci.yml
• pnpm/action-setup@v4 now auto-detects version from packageManager field in package.json

- uses: pnpm/action-setup@v4
-   with:
-     version: 9
+ uses: pnpm/action-setup@v4

Single source of truth: package.json declares pnpm@9.15.9.

Original prompt

Fix this error Error: Error: Multiple versions of pnpm specified:

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