Prepare package for npm publishing as @sourceacademy/conductor

[martin-henz]

This PR updates the repo to include the changes that were necessary for version 0.3.0: https://www.npmjs.com/package/@sourceacademy/conductor/v/0.3.0

Summary

• Rename package from conductor to @sourceacademy/conductor in package.json
• Update yarn.lock to reflect the new scoped package name
• Add .yarnrc.yml to .gitignore to prevent accidentally committing npm credentials

Test plan

• Verify npm publish works with the scoped package name
• Confirm .yarnrc.yml is not tracked by git

🤖 Generated with the help of Claude Code

[martin-henz]

Closing in favour of #2, which has been rebased and cleaned up.