Replace yarn with pnpm as package manager

[devin-ai-integration]

Replace yarn with pnpm as package manager

This PR migrates the magic-js repository from yarn to pnpm as the package manager.

Changes

• Updated package.json to specify [email protected] as the package manager
• Created pnpm-workspace.yaml to replace yarn workspaces configuration
• Updated all scripts to use pnpm instead of yarn
• Updated GitHub workflow files to use pnpm for CI/CD
• Updated README.md with pnpm commands
• Removed yarn-specific files (.yarn directory, .yarnrc.yml, yarn.lock)
• Updated workspace-helpers.ts to use pnpm list command instead of yarn workspaces list

Test Results

• Core magic-sdk package tests are passing (18 tests, 72.15% statement coverage)
• Some React Native package tests are failing with Jest environment configuration errors that may need further investigation

Link to Devin run

https://app.devin.ai/sessions/d4c9e4a2579c45a7a47a5f1877013ae4

Requested by: [email protected]

[devin-ai-integration]

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